Fix typos

Fixes #36 and includes #41.

For #36, the relevant passages are getting pulled from [lib/chai/interface/assert.js](https://github.com/chaijs/chai/blob/master/lib/chai/interface/assert.js#L394-L395). I changed that too and will be creating a PR after this one. I tried to edit stuff so that changes apply cleanly without having to edit the generated html file directly but the repos (and their generated files) seem to be out of sync (I'll try to provide more detail if you want me to). I ended up editing the `out/api/assert/index.html` file directly anyway.

I did the same thing for [lib/chai/core/assertions.js](https://github.com/chaijs/chai/blob/master/lib/chai/core/assertions.js#L18) and `out/api/bdd/index.html`.