Fix GitHub YAML spec issue (branches should be a list)

Author: jb3

.github/workflows/main.yaml

@@ -3,7 +3,8 @@ name: CI
 on:
   pull_request:
   push:
-    branches: main
+    branches:
+      - main
 
 concurrency:
   group: ${{ github.workflow }}-${{ github.ref }}