Fix typos

[xrstf]

I stumbled upon the "expresssions" typo in the generated code of my language and then ran the spellchecker on Pigeon, finding a few other minor typos.

[xrstf]

While I have your attention: thank you for pigeon, it made building my own language such a breeze that I had to read little to no documentation at all, besides the JSON example. And everything just worked.

Thank you for building Pigeon.

[breml]

@xrstf Thanks for your kind words. I happily pass on the compliment further to @mna who laid the foundation.

[mna]

@xrstf Thanks for the kind words, and many thanks to @breml for your great work on this and taking this project to the next level!