Format buildkite PR reporting

Author: nkammah

.buildkite/scripts/build_pr.sh

@@ -86,6 +86,6 @@ buildkite-agent annotate \
      --append \
     "<br>Preview url: ${PREVIEW_URL}"
 
-buildkite-agent meta-data set pr_comment:doc-preview:head "Documentation preview
+buildkite-agent meta-data set pr_comment:doc-preview:head " * Documentation preview
    - 📚 [HTML diff](${PREVIEW_URL}/diff)
    - 📙 [Preview](${PREVIEW_URL})"