Slight BERTScore gallery modifications for accuracy and styling

[baskrahmer]

What does this PR do?

The example was a bit inconsistent (introduction mentions summarization, but actual code not). I made some changes to clarify this.
In addition I removed the temperature kwarg (not needed when do_sample=False) and changes the styling a bit.

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2642.org.readthedocs.build/en/2642/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69%. Comparing base (d8235b2) to head (5e6616d).
Report is 105 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2642   +/-   ##
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  Coverage      69%     69%           
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  Files         316     316           
  Lines       17881   17881           
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  Hits        12331   12331           
  Misses       5550    5550