doc minor typo fix

[marydavis]

Have really been enjoying this course and noticed this today, thought I would help out and submit the fix.

I understand that since this is a PR to a branch, that you will need to close this PR and apply the change yourself :)

[kentcdodds]

@all-contributors please add @marydavis for docs

[allcontributors[bot]]

@kentcdodds

I've put up a pull request to add @marydavis! :tada: