[Docs] Fix redis address scheme

docs/admin/15_config_files.rst

@@ -40,7 +40,7 @@ A configuration file might look like this::
   password = "SuperSecretPassword"
 
   [caching]
-  redis = { enabled = true, address = "127.0.0.1" }
+  redis = { enabled = true, address = "redis://127.0.0.1" }
 
 The `secret_key` setting above defines a single value. The following `http`
 section defines a table (cf. a dictionary) in one way, and you can see the