chore: update MongoDB database name example

- Change the example database name in the .env.example file
- Update from 'ht_api_db' to 'flutter_news_app_api_server_full_source_code_db'
- Ensure users have a clear example of the expected database name

Author: fulleni

.env.example

@@ -4,7 +4,7 @@
 
 # REQUIRED: The full connection string for your MongoDB instance.
 # The application cannot start without a database connection.
-# DATABASE_URL="mongodb://user:password@localhost:27017/ht_api_db"
+# DATABASE_URL="mongodb://user:password@localhost:27017/flutter_news_app_api_server_full_source_code_db"
 
 # REQUIRED: A secure, randomly generated secret for signing JWTs.
 # The application cannot start without this.