Adjust formatting for subheading in challenge

[ndporter]

What is the problem?

The subheading "accessibility adaptation" in the Test Your Working Memory Analog Version challenge box should be formatted to be comparable in size to the main challenge heading (not larger). I tried just changing it from an H2 to H3 but that didn't seem to change it. I'm not sure if there's a native solution using sandpaper or if this is a situation where we should hard-code visual formats (so I didn't make a PR) but it's jarring to run across.

Location of problem (optional)

https://github.com/carpentries/instructor-training/blob/main/episodes/05-memory.md