Documentation/filesystems/proc.txt: fix mistake in the description of Committed_AS

Fix mistake in the description of Committed_AS in kernel documentation.

Signed-off-by: Minto Joseph <mvaliyav@redhat.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>