Fix branch name in deployment workflow

- `$default-branch` is for workflow templates, not actual workflows

Author: taesungh

.github/workflows/deploy.yml

@@ -2,7 +2,7 @@ name: Deploy site to GitHub Pages
 
 on:
   push:
-    branches: [$default-branch]
+    branches: [main]
 
 # Include permissions for GITHUB_TOKEN to deploy to GitHub Pages
 permissions: