Double `Show` Trait in std/fmt documentation

There are 2 `Show` Traits [here](http://doc.rust-lang.org/std/fmt/#traits) that link to the same place.

I'm not certain how this doc works but `Show` is on both [line 423](https://github.com/rust-lang/rust/blob/master/src/libstd/fmt.rs#L423) and 424 on this page.  If the fix is to remove one, then this should be simple.
