Fix some minor issues in the guide.

This PR fixes some minor grammar and spelling issues in the guide. ~~It also adds the missing `{rust}` attribute to a code chunk.~~
That's what it's supposed to refer to, but it's very confusing, because we're talking about numbers. Especially because the sentence didn't earlier say "a zero", it just said "zero". The straightforward reading of the sentence demands that "one" be treated like a number, and that the implication is that passing `1` causes the function to subsequently take the inverse of `0`.