docs (langgraph): Fix bug in the code example (#6114)

- **Description:** Fix the wrong variable name in the example of
`memory.md`.
  - **Dependencies:** NA.

docs/docs/concepts/memory.md

@@ -134,7 +134,7 @@ def update_instructions(state: State, store: BaseStore):
     namespace = ("instructions",)
     current_instructions = store.search(namespace)[0]
     # Memory logic
-    prompt = prompt_template.format(instructions=instructions.value["instructions"], conversation=state["messages"])
+    prompt = prompt_template.format(instructions=current_instructions.value["instructions"], conversation=state["messages"])
     output = llm.invoke(prompt)
     new_instructions = output['new_instructions']
     store.put(("agent_instructions",), "agent_a", {"instructions": new_instructions})
@@ -278,4 +278,4 @@ const items = await store.search(
 ```
 :::
 
-For more information about the memory store, see the [Persistence](persistence.md#memory-store) guide.
+For more information about the memory store, see the [Persistence](persistence.md#memory-store) guide.