fix: list release branch in workflow (#193)

Co-authored-by: John Halloran <jhalloran@oxy.edu>

Author: john-halloran

.github/workflows/matrix-and-codecov-on-merge-to-main.yml

@@ -17,5 +17,6 @@ jobs:
       project: diffpy.stretched-nmf
       c_extension: false
       headless: false
+      release_branch: main
     secrets:
       CODECOV_TOKEN: ${{ secrets.CODECOV_TOKEN }}