fix: sorbet type annotation on client's environ settings

Author: stainless-app[bot]

rbi/lithic/client.rbi

@@ -121,7 +121,7 @@ module Lithic
     sig do
       params(
         api_key: T.nilable(String),
-        environment: NilClass,
+        environment: T.nilable(T.any(Symbol, String)),
         base_url: T.nilable(String),
         max_retries: Integer,
         timeout: Float,