Need same escapes applied to app_diff as prod_diff in deploy-prod workflow

Author: lottspot

.github/workflows/deploy.yml

@@ -52,9 +52,9 @@ jobs:
           if [ -n "$prod_diff" ]; then
             prod_diff_msg=$(cat <<EOF
           To be applied to prod from $prod_manifest_dir:
-          ```diff
+          \`\`\`diff
           $prod_diff
-          ```
+          \`\`\`
 
           EOF)
           fi