xen: Fix one kernel-doc comment

Use colon to separate parameter name from their specific meaning.
silence the warning:

drivers/xen/grant-table.c:1051: warning: Function parameter or member 'nr_pages' not described in 'gnttab_free_pages'
Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=6030Signed-off-by: Yang Li <yang.lee@linux.alibaba.com>
Acked-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/20230731030037.123946-1-yang.lee@linux.alibaba.comSigned-off-by: Juergen Gross <jgross@suse.com>

drivers/xen/grant-table.c

@@ -1044,7 +1044,7 @@ EXPORT_SYMBOL_GPL(gnttab_pages_clear_private);
 
 /**
  * gnttab_free_pages - free pages allocated by gnttab_alloc_pages()
- * @nr_pages; number of pages to free
+ * @nr_pages: number of pages to free
  * @pages: the pages
  */
 void gnttab_free_pages(int nr_pages, struct page **pages)