message: Fix docs deployment for main branch
  One last remnant of the master->main rename.
diff: @@ -68,7 +68,7 @@ jobs:
  - name: mdbook
  run: mdbook build
  - name: rsync
  - if: ${{github.event_name=='push' && github.ref=='refs/heads/master'}}
  + if: ${{github.event_name=='push' && github.ref=='refs/heads/main'}}
  env:
  DEPLOY_HOST: ${{ secrets.DEPLOY_HOST }}
  DEPLOY_PORT: ${{ secrets.DEPLOY_PORT }}
  