When trying to destructure a `Vec` as a slice provide a suggestion

[estebank]

We currently emit the following:

error[E0529]: expected an array or slice, found `Vec<types::GenericBound>`
   --> src/librustdoc/clean/mod.rs:593:28
    |
593 |                     if let [] | [GenericBound::TraitBound(_, hir::TraitBoundModifier::Maybe)] = &bounds {
    |                            ^^ pattern cannot match with input type `Vec<types::GenericBound>`

error[E0529]: expected an array or slice, found `Vec<types::GenericBound>`
   --> src/librustdoc/clean/mod.rs:593:33
    |
593 |                     if let [] | [GenericBound::TraitBound(_, hir::TraitBoundModifier::Maybe)] = &bounds {
    |                                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ pattern cannot match with input type `Vec<types::GenericBound>`

Ideally we should mention the appropriate solution, to use &bounds[..] instead.