Fixes nits in string guide

/cc @huonw 
That said, most instances of this are probably able to take an `Iterator<&str>`... which that also has flexibility problems, e.g. a `Iterator<String>` cannot become an `Iterator<&str>` without collecting to a `Vec`. On the other hand, collecting to a `Vec` is always required to call a `&[...]` function with an `Iterator` so taking an `Iterator<&str>` is still a strict improvement.