Fixes to guides

Fixes wording in the *runtime*, *testing* and *unsafe* guides.
···<date: 2014-08-11, Monday>···<from: Huon Wilson>···

> > @@ -137,7 +137,7 @@ explicitly with, respectively, `value as *const T` and `value as *mut T`).
> >  
> >  Going the opposite direction, from `*const` to a reference `&`, is not
> >  safe. A `&T` is always valid, and so, at a minimum, the raw pointer
> > -`*const T` has to be a valid to a valid instance of type `T`. Furthermore,
> > +`*const T` has to be a pointer to a valid instance of type `T`. Furthermore,
> 
> Maybe this could be reworded to just "has to point to a valid instance".

I concur. What’s the rust-lang policy on these suggestions? Can
you send me a PR so I can include it, or do you prefer me to add
it myself?