Update README of AutoGenBench MATH scenario

[yiranwu0]

Why are these changes needed?

Update README regarding the automated evaluation of the MATH scenario.
Explain how the automatic evaluation works and potential issues.

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 ✅

Project coverage is 36.10%. Comparing base (676e8e1) to head (746e144).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #1891   +/-   ##
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  Coverage   36.10%   36.10%           
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  Files          63       63           
  Lines        6658     6658           
  Branches     1470     1470           
=======================================
  Hits         2404     2404           
  Misses       4056     4056           
  Partials      198      198           

Flag	Coverage Δ
unittests	36.10% <ø> (ø)

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