Minor fix to GHES 15.3 release notes (#56955)

Author: mchammer01

data/release-notes/enterprise-server/3-15/3.yml

@@ -6,7 +6,7 @@ intro: |
 
   {% endwarning %}
   {% warning %}
-  For instances installed on Google Cloud Platform (GCP), hotpatches to GitHub Enterprise Server version `3.15.3` will result in errors being reported in the upgrade log. We recommend hotpatching to a newer 3.15 version instead. [Updated: 2025-03-11]
+  **Warning**: For instances installed on Google Cloud Platform (GCP), hotpatches to GitHub Enterprise Server version `3.15.3` will result in errors being reported in the upgrade log. We recommend hotpatching to a newer 3.15 version instead. [Updated: 2025-03-11]
 
   {% endwarning %}
 sections: