Update standards.rst

According http://symfony.com/blog/new-in-symfony-3-2-runtime-environment-variables

Author: VolCh

contributing/code/standards.rst

@@ -218,7 +218,7 @@ Service Naming Conventions
 
 * The DI alias of the bundle is the first group (e.g. ``fos_user``);
 
-* Use lowercase letters for service and parameter names;
+* Use lowercase letters for service and parameter names (only one exception - %env(VARIABLE_NAME)% syntax in parameters);
 
 * A group name uses the underscore notation.