doc: api_v3.md prototol ->protocol (#62)

Author: pigercc

api_v3.md

@@ -39,7 +39,7 @@ Method
 The client methods returns either a `etcd` object or an `error string`.
 
 ```lua
-local cli, err = require("resty.etcd").new({prototol = "v3"})
+local cli, err = require("resty.etcd").new({protocol = "v3"})
 ```
 
 Please refer the **etcd API documentaion** at - https://github.com/coreos/etcd for more details.
@@ -260,4 +260,4 @@ local res, err = cli:txn(compare, success, nil)
 
 Gets the etcd version info.
 
-[Back to TOP](#api-v3)
+[Back to TOP](#api-v3)