doc/cephadm: improving "Starting the Upgrade"

doc/cephadm: improving "Starting the Upgrade"

This PR (slightly) improves the text in the section "Starting
the Upgrade" in the "Upgrading Ceph" chapter of the cephadm
documentation.

This is a very minor update, and does little but bring the sentences
into agreement with many other sentences that I've already written.
This is done to give the reader an almost tabular sense of what to
expect when looking at our docs.

Signed-off-by: Zac Dover <zac.dover@gmail.com>