add docs to test paths, fix failing doctest

[braingram]

pyproject.toml test paths did not include the docs directory. This limited the number of doctests run during pytest runs (and the CI in this package). Downstream testing in asdf revealed this limitation and one failing doctest.

This PR adds the docs directory to the test paths and fixes the one failing doctest.

Checklist

• added entry in CHANGES.rst (either in Bug Fixes or Changes to API)
• updated relevant tests
• updated relevant documentation
• updated relevant milestone(s)
• added relevant label(s)

[codecov]

Codecov Report

Patch coverage has no change and project coverage change: +0.10 🎉

Comparison is base (9281ec1) 63.95% compared to head (3d1352e) 64.06%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #182      +/-   ##
==========================================
+ Coverage   63.95%   64.06%   +0.10%     
==========================================
  Files         101      101              
  Lines        5565     5565              
==========================================
+ Hits         3559     3565       +6     
+ Misses       2006     2000       -6     

see 1 file with indirect coverage changes

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[hbushouse]

Looks OK to me.