fix(admin): update the API

#### admin:directory_v1
The following keys were changed:
- schemas.DirectoryChromeosdevicesIssueCommandRequest.properties.payload.description

discovery/admin-directory_v1.json

@@ -4139,7 +4139,7 @@
       }
     }
   },
-  "revision": "20201013",
+  "revision": "20201020",
   "rootUrl": "https://www.googleapis.com/",
   "schemas": {
     "Alias": {
@@ -5137,7 +5137,7 @@
           "type": "string"
         },
         "payload": {
-          "description": "The payload for the command, provide it only if command supports it. The following commands support adding payload: - SET_VOLUME: Payload is a JSON object in the form: { \"volume\": 50 }. The volume has to be an integer in the range [0,100].",
+          "description": "The payload for the command, provide it only if command supports it. The following commands support adding payload: - SET_VOLUME: Payload is a stringified JSON object in the form: { \"volume\": 50 }. The volume has to be an integer in the range [0,100].",
           "type": "string"
         }
       },

src/apis/admin/directory_v1.ts

@@ -820,7 +820,7 @@ export namespace admin_directory_v1 {
      */
     commandType?: string | null;
     /**
-     * The payload for the command, provide it only if command supports it. The following commands support adding payload: - SET_VOLUME: Payload is a JSON object in the form: { "volume": 50 }. The volume has to be an integer in the range [0,100].
+     * The payload for the command, provide it only if command supports it. The following commands support adding payload: - SET_VOLUME: Payload is a stringified JSON object in the form: { "volume": 50 }. The volume has to be an integer in the range [0,100].
      */
     payload?: string | null;
   }