Update example to show keys wrapped by user ID

proposals/1756-cross-signing.md

@@ -432,7 +432,8 @@ response:
       }
     }
   },
-  "master_key": {
+  "master_keys": {
+    "@alice:example.com": {
       "user_id": "@alice:example.com",
       "usage": ["master"],
       "keys": {
@@ -443,8 +444,10 @@ response:
           "ed25519:HIJKLMN": "base64+signature+of+master+key"
         }
       }
+    }
   },
-  "self_signing_key": {
+  "self_signing_keys": {
+    "@alice:example.com": {
       "user_id": "@alice:example.com",
       "usage": ["self_signing"],
       "keys": {
@@ -456,6 +459,7 @@ response:
         }
       }
     }
+  }
 }
 ```