Correct semantic use of markup

This one may be on me, so I'll fix this, but there's a semantic distinction between `<i>` and `<em>`

`<em>` is used for emphasis and tools that use semantic information (like screenreaders) will interpret it this way: For example: We should _really_ get to this soon.

`<i>` is used for stylistic italics, such as for species names and, most relevant here, book and journal titles.