Update quickstart.md

[EmmaB]

Why:

To improve the quality of the writing, and thus its readability.

What's being changed:

Tidy up punctuation, remove comma splices, remove unnecessary words.

e.g. the comma splice is removed from this:

Feel free to create a new repository for this Quickstart, you can use it to test this and future xxx workflows.

The new sentence is complete, shorter, and easier to read:

Feel free to create a new repository for this Quickstart to test this and future xxx workflows.

Check off the following:

• I have reviewed my changes in staging. (look for the deploy-to-heroku link in your pull request, then click View deployment)
• For content changes, I have reviewed the localization checklist
• For content changes, I have reviewed the Content style guide for GitHub Docs.

[welcome]

Thanks for opening this pull request! A GitHub docs team member should be by to give feedback soon. In the meantime, please check out the contributing guidelines.

[janiceilene]

@EmmaB Thanks so much for opening a PR! I'll get this triaged for review ✨

[skedwards88]

🚀 Thanks for taking the time to improve these docs! I removed one comma (feel free to comment if you disagree.) I'll get this merged down for you.