Update Flags parameter description in FilterUnloadCallback

Author: RatinCN

windows-driver-docs-pr/ifs/writing-a-filterunloadcallback-routine.md

@@ -22,7 +22,7 @@ typedef NTSTATUS
     );
 ```
 
-The *FilterUnloadCallback* routine has one input parameter, *Flags*, which can be **NULL** or FLTFL\_FILTER\_UNLOAD\_MANDATORY. The filter manager sets this parameter to FLTFL\_FILTER\_UNLOAD\_MANDATORY to indicate that the unload operation is mandatory. For more information about this parameter, see [**PFLT\_FILTER\_UNLOAD\_CALLBACK**](/windows-hardware/drivers/ddi/fltkernel/nc-fltkernel-pflt_filter_unload_callback).
+The *FilterUnloadCallback* routine has one input parameter, *Flags*, which can be **0** or FLTFL\_FILTER\_UNLOAD\_MANDATORY. The filter manager sets this parameter to FLTFL\_FILTER\_UNLOAD\_MANDATORY to indicate that the unload operation is mandatory. For more information about this parameter, see [**PFLT\_FILTER\_UNLOAD\_CALLBACK**](/windows-hardware/drivers/ddi/fltkernel/nc-fltkernel-pflt_filter_unload_callback).
 
 A minifilter driver's *FilterUnloadCallback* routine must perform the following steps: