prevent backslash of "\ifdef" in listing to be rendered without the backslash (PR #46)

modules/directives/pages/conditionals.adoc

@@ -29,8 +29,10 @@ The conditional preprocessor directives determine which lines to add and which o
 
 If you don't want a conditional preprocessor directive to be processed, you must escape it using a backslash.
 
+// the following listing uses attributes to avoid the backslash to be removed before rendering it in HTML
+[source,subs=attributes+]
 ----
-\ifdef::just-an-example[]
+{backslash}ifdef::just-an-example[]
 ----
 
 Escaping the directive is necessary _even if it appears in a verbatim block_ since it's not aware of the surrounding document structure.