Fix link for MongoDBAtlasVectorSearch in README

[Jibola]

Ticket is extraneous

Changes in this PR

Updated link for MongoDBAtlasVectorSearch to the correct documentation. The last one led to a failing link check https://docs.langchain.com/oss/python/integrations/vectorstores/mongodb_atlas

Test Plan

Passes tests

Checklist

Checklist for Author

• Did you update the changelog (if necessary)?
• Is the intention of the code captured in relevant tests?
• If there are new TODOs, has a related JIRA ticket been created?
  - [ ] Has a MongoDB Employee run the patch build of this PR?

• Not needed since it's a link change

Checklist for Reviewer

- [ ] Does the title of the PR reference a JIRA Ticket?

• Ticket is extraneous

• Do you fully understand the implementation? (Would you be comfortable explaining how this code works to someone else?)
• Have you checked for spelling & grammar errors?
• Is all relevant documentation (README or docstring) updated?