Fix typo in apex plugin docstring

[ryanking13]

What does this PR do?

This PR fixes typo in apex plugin docstring

Before submitting

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[codecov]

Codecov Report

Merging #5835 (e20f171) into master (e429f97) will decrease coverage by 0%.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #5835   +/-   ##
======================================
- Coverage      93%     93%   -0%     
======================================
  Files         134     134           
  Lines       10051   10051           
======================================
- Hits         9397    9396    -1     
- Misses        654     655    +1     

[carmocca]

Generated docs link: https://95272-178626720-gh.circle-artifacts.com/0/html/plugins.html

LGTM!