Update docs according to lint-staged v10

Created by: lukyth

According to lint-staged v10 changelog (https://github.com/okonet/lint-staged#v10):

From v10.0.0 onwards any new modifications to originally staged files will be automatically added to the commit. If your task previously contained a git add step, please remove this. The automatic behaviour ensures there are less race-conditions, since trying to run multiple git operations at the same time usually results in an error.

There's no need to specify git add on the config anymore. So it should be removed from the doc as well.