updated "ember server" references to "ember serve"

[StreakInTheSky]

Was going through the tutorial and noticed they were telling us to use the "ember server" command to run the dev server. This was inconsistent with the quickstart and what's documented in the CLI, so I updated it.

Also noticed the CLI says to run "npm start", which is an alias to "ember serve", but just wondering if that should be updated as well.

Thanks!