Documentation for std::env::SplitPaths is slightly wrong

[Here](https://github.com/rust-lang/rust/blob/e543815c85d4ec3ad4df30f4f9eb7933378ad47f/src/libstd/env.rs#l329) `SplitPaths` is described as 

```
An iterator over `Path` instances 
```

But [in fact](https://github.com/rust-lang/rust/blob/e543815c85d4ec3ad4df30f4f9eb7933378ad47f/src/libstd/env.rs#l363) it's an iterator over `PathBuf` instances.
