mobx-6: use toJS()-wrapper since monkey-patching require(mobx).toJS doesn't work

Signed-off-by: Roman <ixrock@gmail.com>

docs/extensions/guides/stores.md

@@ -50,9 +50,9 @@ export class ExamplePreferencesStore extends Store.ExtensionStore<ExamplePrefere
   }
 
   toJSON(): ExamplePreferencesModel {
-    return toJS({
-      enabled: this.enabled
-    });
+    return {
+      enabled: toJS(this.enabled)
+    };
   }
 }
 ```