readme: switch example tag use to 'nautilus'
readme: switch example tag use to 'nautilus'

Switch the example of how to compile go-ceph with tags to refer to
nautilus in light of luminous and mimic being deprecated.
Small wording/grammar changes to try and make the sentence clearer.

Signed-off-by: John Mulligan <eda8ad33169deab44f9fe65847f99e368da6c91b@redhat.com>

ceph/go-ceph,ceph/go-ceph,ceph/go-ceph