Issue in Rust docs for Ty in libsyntax

This isn't a big deal but the documentation is slightly out of date for the `TyFixedLengthVec` variant of `ast::Ty`. 

Reference: http://doc.rust-lang.org/nightly/src/syntax/ast.rs.html#1247.

The comment should read: 

```
/// A fixed length array (`[T; n]`)
```

I will fix this if I have time, otherwise it is a small tweak that someone looking to contribute could make.

cc @steveklabnik 
