fix: Change CI badge to right directory

[stevenhua0320]

@sbillinge Ready to review, Previously I followed diffpy.cmi README.rst to fix, but I forgot to change the directory to diffpy.srxplanar

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 50.00%. Comparing base (f454574) to head (8f2aaed).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #33   +/-   ##
=======================================
  Coverage   50.00%   50.00%           
=======================================
  Files           2        2           
  Lines          18       18           
=======================================
  Hits            9        9           
  Misses          9        9           

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.