Fix pull_request_target if_merged example

Within the workflow, the event object is still `github.event.pull_request`, so the `merged` key is at `github.event.pull_request.merged`. There _is not_ a `github.event.pull_request_target` object. This is also reflected in the table for this section in "Webhook event payload".

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

@@ -899,7 +899,7 @@ on:
 
 jobs:
   if_merged:
-    if: github.event.pull_request_target.merged == true
+    if: github.event.pull_request.merged == true
     runs-on: ubuntu-latest
     steps:
     - run: |