docs: Memory and RAG: add missing backtick for class reference

[roysha1]

Why are these changes needed?

Update Memory and RAG doc to include missing backticks for class references.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.67%. Comparing base (67ebeed) to head (57420d0).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6656   +/-   ##
=======================================
  Coverage   79.67%   79.67%           
=======================================
  Files         231      231           
  Lines       17172    17172           
=======================================
  Hits        13682    13682           
  Misses       3490     3490           

Flag	Coverage Δ
unittests	79.67% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.

[victordibia]

Thanks