doc/cephfs: s/subvolumegroups/subvolume groups

Use the term "subvolume groups" instead of "subvolumegroups" where the
term appears in plain English. The string "subvolumegroups" is correct
in commands, and remains unchanged.

Also add formatting to command output, to make clearer that the output
is indeed output.

Fixes: ceph#57574 (comment)

Signed-off-by: Zac Dover <[email protected]>

Author: zdover23

doc/cephfs/fs-volumes.rst

@@ -228,8 +228,8 @@ following form:
 
 The ``exist`` command outputs:
 
-* "subvolumegroup exists": if any subvolumegroup is present
-* "no subvolumegroup exists": if no subvolumegroup is present
+* ``subvolumegroup exists``: if any subvolumegroup is present
+* ``no subvolumegroup exists``: if no subvolumegroup is present
 
 .. note:: This command checks for the presence of custom groups and not
    presence of the default one. A subvolumegroup-existence check alone is not
@@ -902,7 +902,7 @@ to policies. This can distribute load across MDS ranks in predictable and
 stable ways.  Review :ref:`cephfs-pinning` and :ref:`cephfs-ephemeral-pinning`
 for details on how pinning works.
 
-Run a command of the following form to configure pinning for subvolumegroups:
+Run a command of the following form to configure pinning for subvolume groups:
 
 .. prompt:: bash #