PYTHON-5656: Fixes broken link to aggregation pipeline docs.

[caseyclements]

Summary

Link Check tests are failing in master and PRs.
This could be reproduced locally with just docs-link. Two line change ( one for sync class, the other for the async class).

The failing section is this:

(api/pymongo/asynchronous/database: line 3) broken https://mongodb.com/docs/manual/reference/operator/aggregation-pipeline - 404 Client Error: Not Found for url: https://www.mongodb.com/docs/manual/reference/operator/aggregation-pipeline

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?

Checklist for Reviewer {@primary_reviewer}

• Does the title of the PR reference a JIRA Ticket?
• 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?

Focus Areas for Reviewer (optional)