Fix invalid syntax for pull_request_target example (#1890)

Co-authored-by: Sarah Edwards <skedwards88@github.com>

Author: tomjschuster

content/actions/reference/events-that-trigger-workflows.md

@@ -593,7 +593,8 @@ By default, a workflow only runs when a `pull_request_target`'s activity type is
 For example, you can run a workflow when a pull request has been `assigned`, `opened`, `synchronize`, or `reopened`.
 
 ```yaml
-on: pull_request_target
+on:
+  pull_request_target:
     types: [assigned, opened, synchronize, reopened]
 ```