feat: fixed grammar/formatting of entire document

[cmccormack]

• I have read freeCodeCamp's contribution guidelines.
• My pull request has a descriptive title (not a vague title like Update index.md)
• My pull request targets the master branch of freeCodeCamp.
• None of my changes are plagiarized from another source without proper attribution.
• All the files I changed are in the same world language (for example: only English changes, or only Chinese changes, etc.)
• My changes do not use shortened URLs or affiliate links.

[thecodingaviator]

Suggested change

	[See A Whole Bunch of Amazing Stuff Pseudo Elements Can Do!](https://www.w3schools.com/css/css_pseudo_elements.asp)
	[See A Whole Bunch of Amazing Stuff Pseudo Elements Can Do!](https://css-tricks.com/pseudo-element-roundup/)

I feel this is better and shows more variety

[cmccormack]

I find both of those links to be valid. My goal was not really to modify the content so much as clean everything up - if you'd like to submit a pull request to change that link please feel free!

[raisedadead]

Hi @cmccormack @RandellDawson @thecodingaviator

I am going ahead and merging this article PR out of order. We are testing something on the Guide's client code and would need to trigger a build.

This PR seems to have some jsFiddle links that we would like to test something on.

Thanks.