doc/cephfs/client-auth: description and example are inconsistent

doc/cephfs/client-auth: description and example are inconsistent

According to the path restriction example, the filesystem name
should be cephfs_a, not cephfs.  Converge on cephfs_a to avoid it
being confused with with the pool tag, which is always cephfs.

This was introduced in 160c4bfeb811 ("mon/AuthMonitor: Use new osd
auth caps for ceph fs authorize").

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
(cherry picked from commit 267c00012a6562daba385186908598f348db059a)

Conflicts:
	doc/cephfs/client-auth.rst [ commit e7a7cf429ef3 ("doc:
	  filesystem to file system") not in mimic ]