Added the missing parameter of special_case_swap_possible to docs

[Rocche]

Hey! I noticed that in the description of the tisbury-treasure-hunt challenge there is an error in section 4 - Special Places.

Right now the description states the following:

Implement the special_case_swap_possible function, which takes a treasure (such as #("Amethyst Octopus", #(1, "F"))) and a Place (such as #("Seaside Cottages", #("C", 1))), and returns True for the following combinations:
[...]

So, reading the description I would expect to find the special_case_swap_possible function that accepts only two parameters, but instead in the code the function accepts three parameters, the additional one being desired_treasure.

With that being said, I added the missing parameter to the description of the exercise.

[lpil]

Thank you!!