fix(iamcredentials): update the API

#### iamcredentials:v1
The following keys were changed:
- schemas.SignJwtRequest.properties.payload.description

discovery/iamcredentials-v1.json

@@ -226,7 +226,7 @@
       }
     }
   },
-  "revision": "20200515",
+  "revision": "20200605",
   "rootUrl": "https://iamcredentials.googleapis.com/",
   "schemas": {
     "GenerateAccessTokenRequest": {
@@ -344,7 +344,7 @@
           "type": "array"
         },
         "payload": {
-          "description": "Required. The JWT payload to sign. Must be a serialized JSON object that contains a\nJWT Claim Set. For example: `{\"sub\": \"user@example.com\", \"iat\": 313435}`\n\nIf the claim set contains an `exp` claim, it must be an integer timestamp\nthat is not in the past and at most 12 hours in the future.",
+          "description": "Required. The JWT payload to sign. Must be a serialized JSON object that contains a\nJWT Claims Set. For example: `{\"sub\": \"user@example.com\", \"iat\": 313435}`\n\nIf the JWT Claims Set contains an expiration time (`exp`) claim, it must be\nan integer timestamp that is not in the past and no more than 12 hours in\nthe future.",
           "type": "string"
         }
       },

src/apis/iamcredentials/v1.ts

@@ -197,7 +197,7 @@ export namespace iamcredentials_v1 {
      */
     delegates?: string[] | null;
     /**
-     * Required. The JWT payload to sign. Must be a serialized JSON object that contains a JWT Claim Set. For example: `{"sub": "user@example.com", "iat": 313435}`  If the claim set contains an `exp` claim, it must be an integer timestamp that is not in the past and at most 12 hours in the future.
+     * Required. The JWT payload to sign. Must be a serialized JSON object that contains a JWT Claims Set. For example: `{"sub": "user@example.com", "iat": 313435}`  If the JWT Claims Set contains an expiration time (`exp`) claim, it must be an integer timestamp that is not in the past and no more than 12 hours in the future.
      */
     payload?: string | null;
   }