Change preview branch to 'previews' in workflow

Signed-off-by: John Mertic <[email protected]>

Author: jmertic

.github/workflows/preview.yml

@@ -33,4 +33,5 @@ jobs:
         uses: rossjrw/pr-preview-action@v1
         with:
           source-dir: ./_site/
-          preview-branch: main
+          preview-branch: previews
+          umbrella-dir: previews