mountinfo: fix filters doc
mountinfo: fix filters doc

Commit 994129b042b29f makes Fstype available to filters,
but fails to mention it in the FilterFunc doc. Fix it.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

moby/sys