Iterator documentation lists Box impl twice

[jplatte]

As far as I can tell, these two impls are the same:

• https://doc.rust-lang.org/1.59.0/std/iter/trait.Iterator.html#impl-Iterator-147
• https://doc.rust-lang.org/1.59.0/std/iter/trait.Iterator.html#impl-Iterator-178

The second one doesn't have a [src] link so I don't know where it's coming from.

[jplatte]

@rustbot modify labels: +T-rustdoc