Fix formatting of notebook contributing.md

[jackgerrits]

Why are these changes needed?

Fixes a small formatting issue and a missing :

Related issue number

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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5b217c9) 34.24% compared to head (eab6296) 33.00%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1540      +/-   ##
==========================================
- Coverage   34.24%   33.00%   -1.25%     
==========================================
  Files          42       40       -2     
  Lines        5098     5090       -8     
  Branches     1165     1165              
==========================================
- Hits         1746     1680      -66     
- Misses       3209     3270      +61     
+ Partials      143      140       -3     

Flag	Coverage Δ
unittests	33.00% <ø> (-1.25%)	⬇️

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