Adding carreau as short-term maintainer

[tracykteal]

@ErinBecker @PeterSmyth12 @gtlaflair @juanfung

@Carreau is doing some work on these lessons in preparation to teach them. He's submitted several successful PRs to the lesson already. I've made him a short-term maintainer to facilitate this work. Since these are Alpha, he will make typo type updates directly, but will put any suggested conceptual changes in through a PR.

[Carreau]

Thanks, issues are mostly with layout, and/or reference to the ecology-lesson ;I'll make sure to go through PRs for anything that would be a deeper issue.