Substituted 'yml' to '.yaml' in relevant files

[bazcrown]

This is a small doc fix for
#5533

Resolves #5533

[codecov]

Codecov Report

Merging #5540 into master will decrease coverage by 0.02%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5540      +/-   ##
==========================================
- Coverage   96.11%   96.08%   -0.03%     
==========================================
  Files         117      117              
  Lines       25695    25695              
  Branches     2493     2493              
==========================================
- Hits        24696    24690       -6     
- Misses        695      701       +6     
  Partials      304      304

Impacted Files	Coverage Δ
src/_pytest/doctest.py	94.38% <0%> (-2.25%)	⬇️

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[asottile]

thanks!