UBI: Fix typo in comment

While we are here fix a s/beween/between typo.

CRs-Fixed: 975289
Change-Id: Ib96b93899da82e2d17f1a55637959497fee34e35
Signed-off-by: Richard Weinberger <richard@nod.at>
Acked-by: Brian Norris <computersforpeace@gmail.com>
Git-commit: 4ebb4c9dcbe88a683b083f03176612ee8e87befb
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


Signed-off-by: Nikhilesh Reddy <reddyn@codeaurora.org>