Corrected get_cache_key() signature in docs.

Follow up to b22415214a7bdeaf8ccd7b8b21872038ab865991 and 
cb17f7ca2252265ab4a844e7924cb8ebab4a1a76

docs/ref/utils.txt

@@ -87,7 +87,7 @@ need to distinguish caches by the ``Accept-language`` header.
         Handling an asterisk ``'*'`` according to :rfc:`7231#section-7.1.4` was
         added.
 
-.. function:: get_cache_key(request, key_prefix=None)
+.. function:: get_cache_key(request, key_prefix=None, method='GET', cache=None)
 
     Returns a cache key based on the request path. It can be used in the
     request phase because it pulls the list of headers to take into account