Adjust codespell invocation to only run codespell on .sh, .go, .md

[tstromberg]

Fixes #529

[codecov]

Codecov Report

Merging #530 (eba0ae3) into master (18bc57d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #530   +/-   ##
=======================================
  Coverage   33.61%   33.61%           
=======================================
  Files          44       44           
  Lines        3385     3385           
=======================================
  Hits         1138     1138           
  Misses       2150     2150           
  Partials       97       97           

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