Prior to this change, the CephFS snapshot instructions were not clear
whether users should "mkdir" the ".snap" directory or not.
Update the documentation to clarify that users should make a directory
under .snap with a name of their choice.
Thanks to John Spray <jspray@redhat.com> on ceph-users for this
information.
Signed-off-by: Ken Dreyer <kdreyer@redhat.com>
