ubi: fastmap: Fix typo in comments

There are a typo(dont't) in comments.
Fix it.

Signed-off-by: Zhang Jiaming <jiaming@nfschina.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>