place newlines around div fences

[zkamvar]

This is related to carpentries/pegboard#84. It's a subtle quirk of commonmark (which pegboard uses for parsing) to assume that anything immediately following a list is part of the list (unless it's a higher order markdown element). This quirk was causing the build to fail because I did not account for empty divs in {pegboard} (and I will fix this soon, but this PR will cause those troubles to melt away).

This fixes the formatting issues with the lesson, but there is another problem that will cause the build to fail unrelated to the formatting: the ratdat package needs to be updated in the package cache because it added a new data set (complete_old) that was not available when this lesson was first created.

Normally, if this repo were on one of The Carpentries organisations, then updates to the package cache would arrive in automated pull requests because you would have access to @carpentries-bot, but because you don't have access to the bot, you have two options to fix the situation:

1. (hard mode) Create a SANDPAPER_WORKFLOW token and add it to this repository (see https://carpentries.github.io/sandpaper/articles/automated-pull-requests.html and "Instructions to create a new token" in https://github.com/MCMaurer/Rewrite-R-ecology-lesson/runs/5457683468?check_suite_focus=true
2. (simpler) locally, with an up-to-date version of sandpaper, run sandpaper::update_workflows() and sandpaper::update_cache()

[MCMaurer]

Alright, I went ahead and ran sandpaper::update_github_workflows() and sandpaper::update_cache() and everything seems to have gone swimmingly. I'll go ahead and merge this PR and then push my cache changes. Thanks!!