doc/cephfs: s/mountpoint/mount point/

Change the string "mountpoint" to "mount point" in English-language
strings (as opposed to in commands, where the string "mountpoint"
sometimes appears and is correct).

cf. https://github.com/ceph/ceph/pull/58908#discussion_r1697715486
in which page 345 of The IBM Style Guide is referenced to back up this
change.

Signed-off-by: Zac Dover <zac.dover@proton.me>
(cherry picked from commit 57ed73eaa6945c69059d299ea5310eaac57e2299)