Region to platform in the endpoint URL

[BenFradet]

API ref: https://discussion.developer.riotgames.com/articles/652/riot-games-api-v3.html

Code ref:

• https://github.com/BenFradet/RiotSharp/blob/develop/RiotSharp/RateLimitedRequester.cs#L25
• https://github.com/BenFradet/RiotSharp/blob/develop/RiotSharp/RateLimitedRequester.cs#L48
• etc

every occurrence of region needs to be platform according to this mapping https://developer.riotgames.com/regional-endpoints.html

[JanOuborny]

Addressed in #391. Changing to platform domains must be performed during the migration of each endpoint. Closing.