fix docs langgraph persistence checkpoints md

[sh-gongwenlong]

Overview

Fix typo in import statement: corrected incorrect MemoryServer reference to the valid MemorySaver export from @langchain/langgraph, aligning the code example with the actual library API (eliminates import errors for end users).

Related issues/PRs

• GitHub issue: N/A
• Feature PR: N/A

• Linear issue: N/A
• Slack thread: N/A

Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed

Additional notes

The code example incorrectly referenced MemoryServer (a non-existent export from @langchain/langgraph), which would cause import errors for users implementing the example. This change corrects the reference to MemorySaver—the official, valid export for persistent memory management in LangGraph. No other functional or structural changes are included; this is a pure typo fix to ensure the example is accurate and runnable.