There are a few packages for prettifying HTML. You can find them by searching the Atom package archive:
Or just go to this link: https://atom.io/packages/search?q=prettify
Once you've selected a package that does what you want you can install it by using the command: apm install [package name]
from the command line or install it using the interface in Preferences.
When the package is installed, follow its instructions for how to activate its capabilities.