ci: update pandoc workflow to run on main branch

Update GitHub Actions workflow configuration to run pandoc to vimdoc
conversion on the main branch instead of the canary branch.

This change ensures documentation is properly generated and updated when
changes are merged into the main branch.

.github/workflows/ci.yml

@@ -20,7 +20,7 @@ jobs:
       # Give the default GITHUB_TOKEN write permission to commit and push the changed files back to the repository.
       contents: write
     name: pandoc to vimdoc
-    if: ${{ github.ref == 'refs/heads/canary' }}
+    if: ${{ github.ref == 'refs/heads/main' }}
     steps:
       - uses: actions/checkout@v4
         with: