Fixes nits in string guide

/cc @huonw 
They are actually in the output and can be visible, they're just acting as combining characters and so being rendered on top of the previous one. You can see this in the github diff, a few of the `+`s have a diacritic above/below. This could be somewhat addressed by printing a leading space (i.e. `println!(" {}", l)`), but then the 'normal' codepoints in the output will look strange.