xen/gntdev: gntdev.h: drop a duplicated word

Drop the repeated word "of" in a comment.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Reviewed-by: Juergen Gross <jgross@suse.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: xen-devel@lists.xenproject.org
Link: https://lore.kernel.org/r/20200719003317.21454-1-rdunlap@infradead.org


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