docs: change CI badge to use GH Actions instead of Travis

[agilgur5]

Summary

Migrate the CI badge from Travis to GH Actions as well

• Follow-up to #84

Details

• use the "CI" workflow on the main branch only

  • note that it is case-sensitive, so had to put the workflow name in caps for the badge to work properly
  • basically same as how the previous badge only reported on main

• missed this one tiny detail during the migration

  • did a search this time to make sure I replaced Travis everywhere

[codecov[bot]]

Codecov Report

Merging #85 (9e83201) into main (514dec0) will not change coverage. The diff coverage is n/a.

@@            Coverage Diff            @@
##              main       #85   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files            1         1           
  Lines           75        75           
  Branches         9         9           
=========================================
  Hits            75        75           

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