appFavorites: Fix nonsense condition

The "in" operator has a lower precedence than negation, so we are
actually testing whether the favorites map contains "false".

Add parentheses to get the intended behavior.

Spotted by eslint.

https://gitlab.gnome.org/GNOME/gnome-shell/merge_requests/375

js/ui/appFavorites.js

@@ -153,7 +153,7 @@ class AppFavorites {
     }
 
     _removeFavorite(appId) {
-        if (!appId in this._favorites)
+        if (!(appId in this._favorites))
             return false;
 
         let ids = this._getIds().filter(id => id != appId);