"Updating Atom" tweaks

[rsese]

A couple of small updates to https://flight-manual.atom.io/getting-started/sections/installing-atom/#updating-atom:

• Miscommunication on my part about which sentence to remove in https://github.com/atom/flight-manual.atom.io/pull/513#discussion_r279132977
• Flesh out the section on updating on Linux a tiny bit