* Make property names monospace
Property names as monospace would be more fitting to this challenge.
* Add default value for align-items
According to the standard, the default value for align-items, if it is not specified, is stretch.
Source: https://www.w3schools.com/cssref/css3_pr_align-items.asp
* Add monospace font for code elements
These elements need to be in a monospace font.
<!-- Please follow this checklist and put an x in each of the boxes, like this: [x]. It will ensure that our team takes your pull request seriously. -->
- [x] I have read [freeCodeCamp's contribution guidelines](https://github.com/freeCodeCamp/freeCodeCamp/blob/master/CONTRIBUTING.md).
- [x] My pull request has a descriptive title (not a vague title like `Update index.md`)
- [x] My pull request targets the `master` branch of freeCodeCamp.
- [x] None of my changes are plagiarized from another source without proper attribution.
- [x] My article does not contain shortened URLs or affiliate links.
Co-authored-by: Paul Gamble <pgamble@gmail.com>