doc/cephfs: s/mountpoint/mount point/

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.

This commit alters only English-language text and example commands in
which the string "{mount point}" is meant to be replaced. No commands
meant for cutting-and-pasting have been altered in this commit.

Signed-off-by: Zac Dover <zac.dover@proton.me>