Correct grammar in APIEmpty documentation (#1748)

* Correct grammar in APIEmpty doc

* Generate swagger.v1.json

modules/context/api.go

@@ -37,7 +37,7 @@ type APIValidationError struct {
 	URL     string `json:"url"`
 }
 
-//APIEmpty is a empty response
+//APIEmpty is an empty response
 // swagger:response empty
 type APIEmpty struct{}
 

public/swagger.v1.json

@@ -1269,7 +1269,7 @@
       }
     },
     "empty": {
-      "description": "APIEmpty is a empty response"
+      "description": "APIEmpty is an empty response"
     },
     "error": {
       "description": "APIError is error format response",