fix example in docs of state_schema in create_react_agent

[jokerkeny]

because in

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

langgraph/libs/langgraph/langgraph/prebuilt/chat_agent_executor.py

Line 543 in c0b56bf

and state["is_last_step"]

the AgentState requires is_last_step to have a default value, like False, and IsLastStep can satisfy it.