Change RANKING_URL to the current ranking

[debruijn]

Without this change, the ranking that is pulled is always the one from March 5 2024. With this change, it will use the .../ranking/teams URL that automatically resolves into the current ranking.

Potential alternatives:

• Add a date input to get_ranking() to allow the user to specify which ranking to pull (-> the ranking most recently before / on the given input date). The default would be to pull the current one.
• Move the URL variables into the class as attributes, such that a user could override, either within an instance of the class or by making a child class.

Edit: I now see this implements the request in issue #5