Normalize import path

Summary: Just a small fix to normalise import path to make typescript happy when this code is located in "flipper" folder (e.g. after installing "flipper" package as dependency) rather than "app" folder.

Reviewed By: jknoxville

Differential Revision: D27156167

fbshipit-source-id: 24495d6b1a756b20fadf62eec18bfe6665eb78e2

desktop/app/src/utils/useIsDarkMode.tsx

@@ -7,7 +7,7 @@
  * @format
  */
 
-import {useStore} from '../../../app/src/utils/useStore';
+import {useStore} from './useStore';
 
 /**
  * This hook returns whether dark mode is currently being used.