11-interfaces should use String() instead of ToString

[stellentus]

I saw a lot of students trying to implement ToString() instead of String(). Yes, they should have read the docs. I like the idea of making it a different name to clearly differentiate this example from the standard library stringer, but I think the students would actually be less confused if we used the same name. Sure, many won't realize that we're defining a different (but identical) interface, but I think that's ok.