fix example in docs of state_schema in create_react_agent (#2109)

because in
```python
    def call_model(
        state: AgentState,
        config: RunnableConfig,
    )
...
        if (
            (
                "remaining_steps" not in state
                and state["is_last_step"]
                and has_tool_calls
            )
```

https://github.com/langchain-ai/langgraph/blob/c0b56bf60d84ed435609c35b0691cd0305ceae78/libs/langgraph/langgraph/prebuilt/chat_agent_executor.py#L543
the AgentState requires is_last_step to have a default value, like
`False`, and `IsLastStep` can satisfy it.

---------

Co-authored-by: Vadym Barda <vadym@langchain.dev>

libs/langgraph/langgraph/prebuilt/chat_agent_executor.py

@@ -382,6 +382,8 @@ def create_react_agent(
 
         ```pycon
         >>> from typing import TypedDict
+        >>>
+        >>> from langgraph.managed import IsLastStep
         >>> prompt = ChatPromptTemplate.from_messages(
         ...     [
         ...         ("system", "Today is {today}"),
@@ -392,7 +394,7 @@ def create_react_agent(
         >>> class CustomState(TypedDict):
         ...     today: str
         ...     messages: Annotated[list[BaseMessage], add_messages]
-        ...     is_last_step: str
+        ...     is_last_step: IsLastStep
         >>>
         >>> graph = create_react_agent(model, tools, state_schema=CustomState, state_modifier=prompt)
         >>> inputs = {"messages": [("user", "What's today's date? And what's the weather in SF?")], "today": "July 16, 2004"}