Documentation: Clarify shortcuts and fix grammar (#6821)

I got stuck on the "Getting started" page because I was trying to use
the shortcut `⌘+,` and not `⌘,`. This PR removes the `+`'s to make the
shortcuts clearer to the reader.

Release Notes:

- Improved getting started instructions.

---------

Co-authored-by: Marshall Bowers <elliott.codes@gmail.com>