mm tracing: cleanup Documentation/trace/events-kmem.txt

Clean up typos/grammos/spellos in events-kmem.txt.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Cc: Mel Gorman <mel@csn.ul.ie>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>