Fix incorrect periodic trigger example in multibranch pipeline JCasC … (#2743)

Co-authored-by: Tim Jacomb <[email protected]>

Author: hyderraza18

demos/jobs/multibranch-github.yaml

@@ -17,6 +17,8 @@ jobs:
           }
         }
         triggers {
-          periodic(5)
+          periodicFolderTrigger {
+            interval('5')
+          }
         }
       }