Remove documentation references to region parameter for Stats Usage endpoints


The documentation for Stats Usage endpoints (/stats/usage, /stats/usage_by_month, and /stats/usage_by_service) made reference to an unsupported region query parameter. The documentation has been updated to remove references to this parameter. No changes were made to the API itself.

Prior change: Rust SDK 0.8.1

Following change: AssemblyScript SDK 0.4.2