The [Error trait](https://rust-exercises.com/100-exercises/05_ticket_v2/09_error_trait.html?highlight=supertrait#the-error-trait) chapter states that the reader may recall the supertrait syntax from the [Sized trait](https://rust-exercises.com/100-exercises/04_traits/08_sized) chapter. Actually the syntax is introduced in the [From and Into](https://rust-exercises.com/100-exercises/04_traits/09_from) chapter instead. This PR amends the text accordingly.
Trim errant `s` from `enqueue`'s end in [Bounded vs unbounded channels](https://rust-exercises.com/100-exercises/07_threads/09_bounded.html): `if the producers enqueues messages` -> `if the producers enqueue messages`
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