docs(migration): problem with 'key' and '$key' in migration guide. (#1228)

Author: Borsaliino

docs/version-5-upgrade.md

@@ -45,7 +45,7 @@ constructor(afDb: AngularFireDatabase) {
     const $key = action.payload.key;
     const data = { $key, ...action.payload.val() };
     return data;
-  }).subscribe(item => console.log(item.key));
+  }).subscribe(item => console.log(item.$key));
 }
 ```