doc: Exposing functions -> change to Heading level 3 (#840)

Adapt markdown for configuration.md file, where `Exposing functions` a heading of the same level as `Exposing Constants` and `Exposing Classes`

docs/configuration.md

@@ -346,7 +346,7 @@ return $loader;
 ```
 
 
-##### Exposing functions
+### Exposing functions
 
 The mechanism is very similar to the one used for classes. However since a
 function similar to `class_alias` does not exists for functions, we declare