Shorten ternary operator

Small fix, shorten the ternary operator as it does exactly the same: https://github.com/twigphp/Twig/blob/2.x/doc/templates.rst#other-operators (see ternary)

Author: royklutman

reference/forms/twig_reference.rst

@@ -340,7 +340,7 @@ done by using a public ``vars`` property on the
 .. code-block:: html+twig
 
     <label for="{{ form.name.vars.id }}"
-        class="{{ form.name.vars.required ? 'required' : '' }}">
+        class="{{ form.name.vars.required ? 'required' }}">
         {{ form.name.vars.label }}
     </label>