Fix elvis operator mistake in reference docs

In the given example the expression using the Elvis operator will
return "Nikola Tesla" and not "Mike Tesla".

Issue: SPR-10850

src/reference/docbook/expressions.xml

@@ -980,7 +980,7 @@ StandardEvaluationContext context = new StandardEvaluationContext(tesla);
 
 String name = parser.parseExpression("Name?:'Elvis Presley'").getValue(context, String.class);
 
-System.out.println(name); // Mike Tesla
+System.out.println(name); // Nikola Tesla
 
 tesla.setName(null);