Fix hyperlinks in From impls between Vec and VecDeque
I'd been trying to link them, but apparently actually just added brackets: <https://doc.rust-lang.org/nightly/std/collections/struct.VecDeque.html#impl-From%3CVec%3CT%3E%3E>
~~This reverts commit 5168f5d220d0b30d322f254f51142931a9054056.~~
~~(I'd previously tried to make relative links, but those failed linkcheck because the types are exported at different levels. So just skip the links -- they're already linked in the function signature anyway.)~~
This makes the links now work.