[Documentation] Fix owner_id type (supabase#40251)

Fix owner_id type

If you don't type cast you'll encounter the following error:
```
Error: ERROR: 42883: operator does not exist: text = uuid HINT: No operator matches the given name and argument types. You might need to add explicit type casts. 
```
This should fix it.

Author: Revadike

apps/docs/content/guides/storage/security/ownership.mdx

@@ -32,7 +32,7 @@ on storage.objects
 for delete
 to authenticated
 using (
-    owner_id = (select auth.uid())
+    owner_id = (select auth.uid()::text)
 );
 ```