Update pull_request_assignment.yml: add xiaoxu to PR review list (#12539)

* add xiaoxu to PR review list

* fix

Author: xiaoxuqi-ms

.github/pull_request_assignment.yml

@@ -148,6 +148,13 @@
     reviewers:
       - PhoenixHe-msft
 
+- rule:
+    paths: 
+      - "specification/azurestack/**"
+      - "specification/automation/**"
+    reviewers:
+      - xiaoxuqi-ms
+
 - rule:
     paths:
       - "documentation/**"