docs(storage): minor fix (#3422)

replace storage with storage() in example

docs/storage/usage/index.md

@@ -131,7 +131,7 @@ need to call the `getDownloadURL` method on a reference:
 ```js
 import storage from '@react-native-firebase/storage';
 
-const url = await storage.ref('images/profile-1.png').getDownloadURL();
+const url = await storage().ref('images/profile-1.png').getDownloadURL();
 ```
 
 > Images uploaded manually via the Firebase Console automatically generate a download URL.