fix(docs): fix wrong var name for memory_service (#497)

Use the correct `memory_service` variable instead of the erroneous `session_service`

Author: mohabfekry

docs/sessions/express-mode.md

@@ -92,7 +92,7 @@ instead initialize the memory object without any project or location.
        APP_ID = "your-reasoning-engine-id"
 
        # Project and location are not required when initializing with Vertex Express Mode
-       session_service = VertexAiMemoryBankService(agent_engine_id=APP_ID)
+       memory_service = VertexAiMemoryBankService(agent_engine_id=APP_ID)
        # Generate a memory from that session so the Agent can remember relevant details about the user
        # memory = await memory_service.add_session_to_memory(session)
        ```