In my opinion, an edit should fix most issues with a post, without changing the intent of the post.
Rewriting /dev/sda1 to /dev/sda1 is not worth a edit, but a large code block may be worth an edit to make it readable. If you choose to edit a post, you should attempt to fix most errors in it, not just some. Take the extra minute to read through it and check if there's other things you can fix while you're at it.
This includes fixing most formatting issues. Don't just hilight code, fix for instance lists if the user has trouble inserting a list in the correct way. Use the supported formatting tools.