Fixes nits in string guide

/cc @huonw 
(Woah, Safari is doing a bad job.)

The best example of the problems is the grapheme section below, I find it hard to believe that the combining characters shouldn't be over their main letter.

![misplaced](https://cloud.githubusercontent.com/assets/1203825/4380872/5bedaefc-436e-11e4-8c05-f1377441fc78.png)

It may just be an issue of fonts, but we could try to tweak the example so it's slightly more portable.

Also, I'm most concerned about this when actually rendered, since that's how most people will be viewing it. @steveklabnik could you put up a `rustdoc`'d version somewhere?