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=6030
Signed-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.com
Signed-off-by: Juergen Gross <jgross@suse.com>