docs: Issues with spelling and grammar in list intro sentence

[alissast]

The following sentence contains a number of stylistic issues and errors: "Thiss documnt explans how you can forked repo and start work on its. please follow thse steps:"

We should probably correct it to something like "This document explains how to fork a repository and start working on it. Follow these steps:"

(Following the Google developer documentation style guide, we should spell out "repository" in full.)