fix: move pull request branch deletion setting (#3476)

.asf.yaml

diff --git a/.asf.yaml b/.asf.yaml
index 57bde43..dffd9c9 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -24,8 +24,8 @@
     discussions: true
   description: A graph database that supports more than 100+ billion data, high performance and scalability (Include OLTP Engine & REST-API & Backends)
   homepage: https://hugegraph.apache.org
-  del_branch_on_merge: true
   pull_requests:
+    del_branch_on_merge: true
     creation_cap:
       enabled: true
       max_open_pull_requests: 5