fix: event reference in component _handleKeyDown

Fix 'ReferenceError: event is not defined' in _handleKeyDown caused by not using '$event' parameter

Author: steven-hadfield

src/lib/dl-date-time-picker/dl-date-time-picker.component.ts

@@ -415,7 +415,7 @@ export class DlDateTimePickerComponent<D> implements OnChanges, OnInit, ControlV
 
       this.focusActiveCell();
       // Prevent unexpected default actions such as form submission.
-      event.preventDefault();
+      $event.preventDefault();
     }
   }