Fixes nits in string guide

/cc @huonw 
> it's still true that this is not what Str is designed for right now

What is it designed for now? (If this is not what it's designed for, the purpose of the trait has changed since it was added. There were explicitly places where we were taking `&[&str]` or `&[~str]`, forcing unnecessary allocations, introducing the `Str` trait allowed this to be avoided.)