fixed attributes -> getAttributes in sample code (PR asciidoctor#458)

to match AsciidoctorJ 1.5.4 API

README.adoc

@@ -966,7 +966,7 @@ public class TerminalCommandTreeprocessor extends Treeprocessor { // <1>
 
   public Block convertToTerminalListing(Block block) {
 
-    Map<String, Object> attributes = block.attributes();
+    Map<String, Object> attributes = block.getAttributes();
     attributes.put("role", "terminal");
     StringBuilder resultLines = new StringBuilder();