String::as_mut_str should have a doc example

[frewsxcv]

https://doc.rust-lang.org/nightly/std/string/struct.String.html#method.as_mut_str

For this one, ideally the example would demonstrate mutation.

If anyone anyone wants to work on this, just mention it in a comment to prevent duplicate work. If you need any help or have any questions, don't hesitate to ping me :)

[tommyip]

I will work on this.