Correct README command examples for chess game sample.

[trevor211]

Why are these changes needed?

Fix outdated script references from chess_game.py to main.py

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 76.64%. Comparing base (8f58e47) to head (9ad7d5c).
Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6008   +/-   ##
=======================================
  Coverage   76.64%   76.64%           
=======================================
  Files         191      191           
  Lines       13201    13201           
=======================================
  Hits        10118    10118           
  Misses       3083     3083           

Flag	Coverage Δ
unittests	76.64% <ø> (ø)

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