Missing backticks breaking documentation in groupchat.last_speaker

[HenryKobin]

Why are these changes needed?

There are missing backticks in docstring causing the generated reference documentation formatting to break.

Related issue number

N/A

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation 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.

[marklysze]

Changes look good, thanks @HenryKobin!

Before:

After:

[marklysze]

Good to go :), @qingyun-wu / @sonichi if you can assist with merge :)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 20.44%. Comparing base (6279247) to head (d26e59f).
Report is 85 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #3357       +/-   ##
===========================================
- Coverage   32.90%   20.44%   -12.46%     
===========================================
  Files          94       95        +1     
  Lines       10235    10661      +426     
  Branches     2193     2278       +85     
===========================================
- Hits         3368     2180     -1188     
- Misses       6580     8326     +1746     
+ Partials      287      155      -132     

Flag	Coverage Δ
unittests	20.43% <ø> (-12.47%)	⬇️

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