mm/Kconfig: fix spelling mistake "whats" -> "what's"

There is a spelling mistake in the Kconfig help text. Fix it.

Link: https://lkml.kernel.org/r/20201217172717.58203-1-colin.king@canonical.com


Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>