Clean up section where Markdown gets parsed oddly

Hi WriteFreely team! I'm setting up a test instance and as such have been deep in your documentation for the last few days, and I thought I'd contribute some small changes where I see issues.

This issue can be seen on the live site here: https://writefreely.org/docs/v0.13.1/writer/writing.md

The final section on how to produce code blocks in markdown gets parsed oddly/doesn't show the intended syntax highlighting when it makes it onto the page (markdown previews render it as intended though). I removed the <code> tags and tested the markdown on my WF instance, and it should now render correctly.

Thanks for all your work! You'll probably see a few of these small documentation pulls from me.

writefreely / documentation

Clean up section where Markdown gets parsed oddly #40