raid: md_p.h: drop duplicated word in a comment

Drop the doubled word "the" in a comment.

Signed-off-by: Randy Dunlap <rdunlap@infradead.org>
Cc: Song Liu <song@kernel.org>
Cc: linux-raid@vger.kernel.org
Signed-off-by: Song Liu <songliubraving@fb.com>