Deal with backticks in data for exercises/Graphing-adult-vs-newborn-size-R.md

[garezana]

How could the content be improved?

Mammal_lifehistories_v2.txt has parenthesis in some column names that require backticks to be used. Telling learnings how to deal with them, renaming them as part of the given data in the exercise, or providing data with already altered column names might benefit learners.

Which part of the content does your suggestion apply to?

https://github.com/datacarpentry/semester-biology/blob/main/exercises/Graphing-adult-vs-newborn-size-R.md