Fix elvis operator example

Closes gh-905

src/asciidoc/core-expressions.adoc

@@ -1113,7 +1113,7 @@ Instead you can use the Elvis operator, named for the resemblance to Elvis' hair
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 	ExpressionParser parser = new SpelExpressionParser();
 
-	String name = parser.parseExpression("null?:'Unknown'").getValue(String.class);
+	String name = parser.parseExpression("name?:'Unknown'").getValue(String.class);
 
 	System.out.println(name); // 'Unknown'
 ----