Minor fix

Author: javiereguiluz

reference/forms/types/options/help.rst.inc

@@ -6,6 +6,6 @@ help
 Allows you to define a help message for the form field, which by default is
 rendered below the field::
 
-    $builder->add('zip_code', null, [
+    $builder->add('zipCode', null, [
         'help' => 'The ZIP/Postal code for your credit card\'s billing address.',
     ]);