Fixes nits in string guide

/cc @huonw 
@huonw, ah I see; I had missed that.

Even so, bounding by `Str` seems a bit unfortunate somehow -- I think because it seems a bit ad hoc/overly specific.

I actually wonder if `Borrow` is the more general concept lurking here? I.e., maybe you want to take `&[T]` where `T: Borrow<str>`? Then the same solution would work for vectors (and eventually other sliceable types).