fix: improve README table format

Author: hsluoyz

README.md

@@ -193,19 +193,19 @@ https://casbin.org/docs/benchmark
 
 ## Examples
 
-Model | Model file | Policy file
-----|------|----
-ACL | [basic_model.conf](https://github.com/casbin/casbin/blob/master/examples/basic_model.conf) | [basic_policy.csv](https://github.com/casbin/casbin/blob/master/examples/basic_policy.csv)
-ACL with superuser | [basic_model_with_root.conf](https://github.com/casbin/casbin/blob/master/examples/basic_with_root_model.conf) | [basic_policy.csv](https://github.com/casbin/casbin/blob/master/examples/basic_policy.csv)
-ACL without users | [basic_model_without_users.conf](https://github.com/casbin/casbin/blob/master/examples/basic_without_users_model.conf) | [basic_policy_without_users.csv](https://github.com/casbin/casbin/blob/master/examples/basic_without_users_policy.csv)
-ACL without resources | [basic_model_without_resources.conf](https://github.com/casbin/casbin/blob/master/examples/basic_without_resources_model.conf) | [basic_policy_without_resources.csv](https://github.com/casbin/casbin/blob/master/examples/basic_without_resources_policy.csv)
-RBAC | [rbac_model.conf](https://github.com/casbin/casbin/blob/master/examples/rbac_model.conf)  | [rbac_policy.csv](https://github.com/casbin/casbin/blob/master/examples/rbac_policy.csv)
-RBAC with resource roles | [rbac_model_with_resource_roles.conf](https://github.com/casbin/casbin/blob/master/examples/rbac_with_resource_roles_model.conf)  | [rbac_policy_with_resource_roles.csv](https://github.com/casbin/casbin/blob/master/examples/rbac_with_resource_roles_policy.csv)
-RBAC with domains/tenants | [rbac_model_with_domains.conf](https://github.com/casbin/casbin/blob/master/examples/rbac_with_domains_model.conf)  | [rbac_policy_with_domains.csv](https://github.com/casbin/casbin/blob/master/examples/rbac_with_domains_policy.csv)
-ABAC | [abac_model.conf](https://github.com/casbin/casbin/blob/master/examples/abac_model.conf)  | N/A
-RESTful | [keymatch_model.conf](https://github.com/casbin/casbin/blob/master/examples/keymatch_model.conf)  | [keymatch_policy.csv](https://github.com/casbin/casbin/blob/master/examples/keymatch_policy.csv)
-Deny-override | [rbac_model_with_deny.conf](https://github.com/casbin/casbin/blob/master/examples/rbac_with_deny_model.conf)  | [rbac_policy_with_deny.csv](https://github.com/casbin/casbin/blob/master/examples/rbac_with_deny_policy.csv)
-Priority | [priority_model.conf](https://github.com/casbin/casbin/blob/master/examples/priority_model.conf)  | [priority_policy.csv](https://github.com/casbin/casbin/blob/master/examples/priority_policy.csv)
+ Model                     | Model file                                                                                                                       | Policy file
+---------------------------|----------------------------------------------------------------------------------------------------------------------------------|----------------------------------------------------------------------------------------------------------------------------------
+ ACL                       | [basic_model.conf](https://github.com/casbin/casbin/blob/master/examples/basic_model.conf)                                       | [basic_policy.csv](https://github.com/casbin/casbin/blob/master/examples/basic_policy.csv)                                       
+ ACL with superuser        | [basic_model_with_root.conf](https://github.com/casbin/casbin/blob/master/examples/basic_with_root_model.conf)                   | [basic_policy.csv](https://github.com/casbin/casbin/blob/master/examples/basic_policy.csv)
+ ACL without users         | [basic_model_without_users.conf](https://github.com/casbin/casbin/blob/master/examples/basic_without_users_model.conf)           | [basic_policy_without_users.csv](https://github.com/casbin/casbin/blob/master/examples/basic_without_users_policy.csv)
+ ACL without resources     | [basic_model_without_resources.conf](https://github.com/casbin/casbin/blob/master/examples/basic_without_resources_model.conf)   | [basic_policy_without_resources.csv](https://github.com/casbin/casbin/blob/master/examples/basic_without_resources_policy.csv)
+ RBAC                      | [rbac_model.conf](https://github.com/casbin/casbin/blob/master/examples/rbac_model.conf)                                         | [rbac_policy.csv](https://github.com/casbin/casbin/blob/master/examples/rbac_policy.csv)
+ RBAC with resource roles  | [rbac_model_with_resource_roles.conf](https://github.com/casbin/casbin/blob/master/examples/rbac_with_resource_roles_model.conf) | [rbac_policy_with_resource_roles.csv](https://github.com/casbin/casbin/blob/master/examples/rbac_with_resource_roles_policy.csv)
+ RBAC with domains/tenants | [rbac_model_with_domains.conf](https://github.com/casbin/casbin/blob/master/examples/rbac_with_domains_model.conf)               | [rbac_policy_with_domains.csv](https://github.com/casbin/casbin/blob/master/examples/rbac_with_domains_policy.csv)
+ ABAC                      | [abac_model.conf](https://github.com/casbin/casbin/blob/master/examples/abac_model.conf)                                         | N/A
+ RESTful                   | [keymatch_model.conf](https://github.com/casbin/casbin/blob/master/examples/keymatch_model.conf)                                 | [keymatch_policy.csv](https://github.com/casbin/casbin/blob/master/examples/keymatch_policy.csv)
+ Deny-override             | [rbac_model_with_deny.conf](https://github.com/casbin/casbin/blob/master/examples/rbac_with_deny_model.conf)                     | [rbac_policy_with_deny.csv](https://github.com/casbin/casbin/blob/master/examples/rbac_with_deny_policy.csv)
+ Priority                  | [priority_model.conf](https://github.com/casbin/casbin/blob/master/examples/priority_model.conf)                                 | [priority_policy.csv](https://github.com/casbin/casbin/blob/master/examples/priority_policy.csv)
 
 ## Middlewares