ci: use main branch instead of master (#515)

Author: chambo-e

.github/workflows/ci.yml

@@ -34,7 +34,7 @@ jobs:
   deploy:
     runs-on: ubuntu-20.04
     needs: [test]
-    if: github.ref == 'refs/heads/master'
+    if: github.ref == 'refs/heads/main'
     steps:
     - uses: actions/[email protected]
       with: