Markdown - improve example content

[CopyJosh]

The Markdown component example is not displayed properly in the docs.

The CONTENT object should probably be:

// src/screens/Markdown.js

const CONTENT = `{\`
 ## Grommet **heart**s markdown
 
 Favorite thing, [link](https://twitter.com/grommet_io)
\`}`;

Currently:

Improved: