Explicitly mention that APIResource.request was removed (#1615)

Author: helenye-stripe

README.md

@@ -347,7 +347,7 @@ If you:
 
 - would like to send a request to an undocumented API (for example you are in a private beta)
 - prefer to bypass the method definitions in the library and specify your request details directly,
-- used the method `Stripe::APIResource.request(...)` to specify your own requests, which will soon be broken
+- used the method `Stripe::APIResource.request(...)` to specify your own requests, which was removed in v13+
 
 you can now use the `raw_request` method on `StripeClient`.