docs: fix cephfs-mirror documentation
docs: fix cephfs-mirror documentation

The steps to configure the peers are detailed in the CephFilesystem
section. Only the CephFilesystem is holding the peer configuration, not
the CephFilesystemMirror which only controls the bootstrap of the
daemon.

Signed-off-by: Sébastien Han <62d754cc350e84d3b1c32ae79f976f5348e74a40@redhat.com>

rook/rook,rook/rook,rook/rook,rook/rook,rook/rook