`docs`: fix link to `VecDeque`

[tertsdiepraam]

The link to std::collections::VecDeque was invalid because std was not in scope, but VecDeque was in scope via alloc so it didn't need an explicit link. This made rustdoc fail with the following error:

error: unresolved link to `std::collections::VecDeque`
 --> src/with_alloc/vecdeque.rs:8:42
  |
8 | /// Wrapper of the built-in [`VecDeque`](std::collections::VecDeque) struct
  |                                          ^^^^^^^^^^^^^^^^^^^^^^^^^^ no item named `std` in scope
  |
note: the lint level is defined here
 --> src/lib.rs:3:9
  |
3 | #![deny(warnings)]
  |         ^^^^^^^^
  = note: `#[deny(rustdoc::broken_intra_doc_links)]` implied by `#[deny(warnings)]`

error: could not document `ringbuffer`

[jdonszelmann]

Thanks!