[security] Add missing serverless mapped page (elastic#1026)

Add missing serverless mapped page.

Author: colleenmcginnis

reference/data-analysis/machine-learning/ootb-ml-jobs-siem.md

@@ -1,6 +1,7 @@
 ---
 mapped_pages:
   - https://www.elastic.co/guide/en/security/current/prebuilt-ml-jobs.html
+  - https://www.elastic.co/guide/en/serverless/current/security-prebuilt-ml-jobs.html
   - https://www.elastic.co/guide/en/machine-learning/current/ootb-ml-jobs-siem.html
 applies_to:
   stack: all