Small typo in noise.rs benchmark

[nsf]

As I'm the original author of it, I was wondering for a looooong time, why rust produces a slightly different result, than all other languages. It seemed like a proper ASCII perlin noise, but at the same time it was different for sure.

Found the bug, typo on this line:
https://github.com/mozilla/rust/blob/master/src/test/bench/noise.rs#L29

Should be: grad.x * sp.x + grad.y * sp.y

I'm not familiar with rust's workflow, so I guess it will be easier for someone who is to just commit that fix. Hence I'm not creating a pull request.