Fix comment.

javascript/ql/src/experimental/Security/CWE-094/UntrustedCheckout.ql

@@ -53,7 +53,7 @@ class ProbableJob extends Actions::Job {
 }
 
 /**
- * on: pull_request_target
+ * The `on: pull_request_target`.
  */
 class ProbablePullRequestTarget extends Actions::On, YamlMappingLikeNode {
   ProbablePullRequestTarget() {