Fix typo in flattening-rectnd-beta-ppt's proof

[jonaprieto]

This PR brings back to life an abandoned PR fixing some typos:

• https://github.com/HoTT/book/pull/1001

I had to open this one, because, for some reason, I deleted the branch for the old PR and couldn't continue there.

The CI is failing, same error as:

• https://github.com/HoTT/book/issues/1139