Use systemctl enable --now shortcut in docs

[jeffgdotorg]

Change Type

Addition

Area

Installation/upgrade

Proposed Changes

For a few years now, the shorthand syntax systemctl enable --now [unit] (which combines enable and start into a single incantation) has been available in major Linux distributions. I think it's safe to update our installation docs to be just a tiny bit shorter.

[jeffgdotorg]

Proposed change set:
develop...jeffgdotorg:netbox:develop