Fix incorrect periodic trigger example in multibranch pipeline JCasC demo (#2731)

Author: hyderraza18

demos/jobs/multibranch-github.yaml

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