fix: incorrect PostgresStore import

[PhantomInTheWire]

Overview

PostgresStore is no longer in @langchain/langgraph-checkpoint-postgres/ but in @langchain/langgraph-checkpoint-postgres/store

Type of change

Type: : Fix outdated content
The public api has changed slightly

Related issues/PRs

None

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
• I have gotten approval from the relevant reviewers
• (Internal team members only / optional) I have created a preview deployment using the Create Preview Branch workflow

[PhantomInTheWire]

@lnhsingh @katmayb can you take a look at this when you get a chance? thanks.