docs: add IngressClassName to ApisixRoute spec (#2246)

Author: kayx23

docs/en/latest/references/apisix_route_v2.md

@@ -36,6 +36,7 @@ The table below describes each of the attributes in the spec. The fields `apiVer
 
 | Attribute                            | Type               | Description                                                                                                                                                                               |
 | ------------------------------------ | ------------------ | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| ingressClassName                                 | string              | Name of the ingress class. Used to target a specific controller that should implement the resource.                                                                                                                                                                         |
 | http                                 | array              | HTTP Route rules.                                                                                                                                                                         |
 | http[].name                          | string (required)  | Route rule name.                                                                                                                                                                          |
 | http[].priority                      | integer            | Route priority. Used to determined which Route to use when multiple routes contain the same URI. Large number means higher priority.                                                      |