Fix reference to supertrait syntax in 09_error_trait.md (#124)

Fix reference to supertrait syntax in 09_error_trait.md (#124)

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.