Fix typo in 05_ticket_v2/09_error_trait/[...] instructions (#13)

Fix typo in 05_ticket_v2/09_error_trait/[...] instructions (#13)

The commented instructions of the exercise ask to use `No description provided` for the description field when calling `easy_ticket` with an invalid description argument. However, the unit test of that behavior compares the description to `Description not provided` instead.

Previous exercises in that part also use `Description not provided`.

Update the exercise 05_ticket_v2/09_error_trait instructions to use `Description not provided` as an expected default  description value when calling `easy_ticket` with no description instead of `No description provided`.