ReduceLROnPlateau doc fixed

[KevinMathewT]

What does this PR do?

Fixes documentation here about ReduceLROnPlateau

Fixes #4454

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[codecov]

Codecov Report

Merging #4459 into master will increase coverage by 2%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #4459    +/-   ##
=======================================
+ Coverage      91%     93%    +2%     
=======================================
  Files         113     113            
  Lines        8396    8191   -205     
=======================================
- Hits         7654    7623    -31     
+ Misses        742     568   -174