Updated the default cache key transformation in documentation.

Follow up to 6c69de80bd.

docs/topics/cache.txt

@@ -1031,7 +1031,7 @@ key version to provide a final cache key. By default, the three parts
 are joined using colons to produce a final string::
 
     def make_key(key, key_prefix, version):
-        return ':'.join([key_prefix, str(version), key])
+        return '%s:%s:%s' % (key_prefix, version, key)
 
 If you want to combine the parts in different ways, or apply other
 processing to the final key (e.g., taking a hash digest of the key