fix: Update SQLite database URL for async connection

The example is broken without the fix.

Co-authored-by: Liang Wu <wuliang@google.com>
PiperOrigin-RevId: 843424168

Author: wuliang229

contributing/samples/migrate_session_db/main.py

@@ -32,7 +32,7 @@
 async def main():
   app_name = 'migrate_session_db_app'
   user_id_1 = 'user1'
-  session_service = DatabaseSessionService('sqlite:///./sessions.db')
+  session_service = DatabaseSessionService('sqlite+aiosqlite:///./sessions.db')
   artifact_service = InMemoryArtifactService()
   runner = Runner(
       app_name=app_name,