changed branchname for codecov GH action

Author: petercorke

.github/workflows/master.yml

@@ -60,7 +60,7 @@ jobs:
         #pytest --cov=spatialmath --cov-report xml
         coverage report
     - name: upload coverage to Codecov
-      uses: codecov/codecov-action@master
+      uses: codecov/codecov-action@main
       with:
         file: ./coverage.xml
   sphinx: