[3.0.x] Corrected get_cache_key() signature in docs.

Follow up to b224152 and
cb17f7c
Backport of d82d2d4 from master

docs/ref/utils.txt

@@ -82,7 +82,7 @@ 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