Documentation fix for method last_valid_index

[gabrielclow]

Documentation was showing "Return index for first non-NA/null value" for both first_valid_index and last_valid_index methods. Fixed by switching "first" to "last"

• [ X ] closes Minor documentation issue on pd.DataFrame.last_valid_index() #18564
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #18681 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18681      +/-   ##
==========================================
- Coverage   91.59%   91.57%   -0.02%     
==========================================
  Files         153      153              
  Lines       51257    51257              
==========================================
- Hits        46949    46941       -8     
- Misses       4308     4316       +8

Flag	Coverage Δ
#multiple	89.44% <ø> (ø)	⬆️
#single	40.68% <ø> (-0.11%)	⬇️

Impacted Files	Coverage Δ
pandas/core/frame.py	97.81% <ø> (-0.1%)	⬇️
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/indexes/datetimes.py	95.59% <0%> (-0.1%)	⬇️
pandas/util/testing.py	82.01% <0%> (+0.19%)	⬆️

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[jreback]

thanks!