fix: modify notifications in .asf.yaml (#24)

diff --git a/.asf.yaml b/.asf.yaml
index 5d12ba6..fa7aa40 100644
--- a/.asf.yaml
+++ b/.asf.yaml
@@ -44,7 +44,7 @@
     merge:   false
     rebase:  false
 
-  notifications:
-    commits:      notifications@apisix.apache.org
-    issues:       notifications@apisix.apache.org
-    pullrequests: notifications@apisix.apache.org
+notifications:
+  commits:      notifications@apisix.apache.org
+  issues:       notifications@apisix.apache.org
+  pullrequests: notifications@apisix.apache.org