Shorten docstring (for CLI compat)

[mathematicalmichael]

CLI only shows first line, this helps the summary fit inside

What does this PR do?

updates a docstring to avoid sentence cut-off in Lightning CLI

No issue - not worth bringing one up IMO for such a minor change.

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---

📚 Documentation preview 📚: https://pytorch-lightning--19356.org.readthedocs.build/en/19356/

[codecov]

Codecov Report

Merging #19356 (a101fd8) into master (1a59097) will decrease coverage by 35%.
The diff coverage is n/a.

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #19356      +/-   ##
==========================================
- Coverage      84%      49%     -35%     
==========================================
  Files         448      440       -8     
  Lines       37854    37701     -153     
==========================================
- Hits        31623    18359   -13264     
- Misses       6231    19342   +13111     

[mathematicalmichael]

pardon the following attempts to get this playing nice with your linter (am editing through github directly).