fix misspellings

[AndrewKvalheim]

Noticed another typo while doing the exercises so I ran a spell check.

[coriolinus]

Looks great, thanks for doing this!

Because this PR technically changes some code, I'll let it sit for a while in case other maintainers have things to say, but unless someone objects, I'll plan to merge this tomorrow. If in two days I have not yet done so, please feel free to ping me a reminder.

[coriolinus]

I'm very sorry--at the time of my approval yesterday, I hadn't realized that Travis wouldn't pass. I forgot that we also check READMEs; it's my fault for prematurely approving. There are four instances where it found unexpected changes:

• crypto-square: the README is generated from the problem specifications; we can't merge this while upstream differs. Please either open an issue upstream to fix this or revert this change.
• dominoes: same issue: the problem is upstream. Please open an issue upstream to fix it or revert this change.
• simple-cipher: Again, the root problem lies upstream. Please either open an issue upstream or revert this change.
• luhn-trait: this is a Rust-specific exercise, so we can fix it in this track. In this case, you'll need to also apply the fix to .meta/metadata.yml in order for Travis to approve.

Reversion will happen on your schedule, but isn't ideal. Opening an issue upstream to fix those spelling errors would be preferable, but will take longer. In either case, I won't merge until Travis passes.

I apologize again for approving prematurely.

[AndrewKvalheim]

Whoops, thanks. The only test I ran was check-exercises.sh.

[coriolinus]

Thanks @AndrewKvalheim for doing this work, and particularly for following through and writing the upstream PR as well!