Fix typos in Documentation/unaligned-memory-access.txt

This patch deletes a couple of superfluous word occurrences in the
document Documentation/unaligned-memory-access.txt.

Thanks to Sebastien Dugue for the remark about English usage.

Signed-off-by: Dmitri Vorobiev <dmitri.vorobiev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>