fix typo

According to https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#check_suite
the key is "types", not "type".

.github/workflows/cirrus-notify.yml

@@ -1,6 +1,6 @@
 on:
   check_suite:
-    type: ['completed']
+    types: ['completed']
 name: Cirrus CI failure notification
 
 permissions: