thp: fix khugepaged defrag tunable documentation

Commit e27e6151 ("mm/thp: use conventional format for boolean
attributes") changed

  /sys/kernel/mm/transparent_hugepage/khugepaged/defrag

to be tuned by using 1 (enabled) or 0 (disabled) instead of "yes" and
"no", respectively.

Update the documentation.

Signed-off-by: David Rientjes <rientjes@google.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>