Clear up punctuation

Thanks for the spot, Learner_.

Here's what was involved:
0) Have Git installed and running, be in my DDA working folder.
1) Make a new branch from my master (`git checkout master, git checkout -b typo`: creates the typo branch from master)
2) Open data/json/mutations.json in my text editor, make the changes shown, save.  I now have a changed file and Git will notice this fact.
3) Commit that change.  I use the GUI for that, so head over there, write up the commit title and message, then hit Commit.
4) Publish the change: GUI territory for me, so just hit the Publish button.  This uploads the "typo" branch to my forked repo on the Git servers, KA101/Cataclysm-DDA.
5) Go into my web browser and PR the typo branch, which (because I'd just published it) Github conveniently makes a button to do.  Leads to this screen, where I'm typing up the PR intro post.  I'm gonna finish this sentence and hit the Create pull request button now.