Fixes nits in string guide

/cc @huonw 
This is actually a pattern that we have been discouraging in the past due to the `Str` trait not quite being what you'd expect, as well as not being quite designed for this purpose. The case of a `&[String]` vs `&[&str]` is a little different, but in general if you're using a string and you don't want ownership you should *always* use `&str`. 

Right now the ergonomics aren't quite where they could be, but there are a few RFCs open about the auto-borrowing issue which should help here.

cc @aturon, do you think something like this should be in the guide for now?