Clarify difference between `adonis serve` with and without --dev flag

[benknight]

adonis serve --help shows:

--dev Start development server

As far as I can gather what this actually means is it will watch files for changes while adonis serve (without the flag) won't

[thetutlage]

Yes. The --dev flag means a file watcher will monitor for file changes. I am happy to accept the PR to improve the flag description :)