Documentation/filesystems/vfs.txt: fix ->repeasepage() description

->releasepage() does not remove the page from the mapping.

Acked-by: Neil Brown <neilb@suse.de>
Cc: Trond Myklebust <Trond.Myklebust@netapp.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>