fix: update existing preview comment instead of creating new one (#94)

Co-authored-by: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Co-authored-by: Catherine Deskur <catherine@buildwithfern.com>

Author: devin-ai-integration[bot]

.github/workflows/preview-docs.yml

@@ -30,3 +30,5 @@ jobs:
         uses: thollander/actions-comment-pull-request@v2.4.3
         with:
           filePath: preview_url.txt
+          comment_tag: preview-docs
+          mode: upsert