fixed bug: vets.html did not display properly

src/main/java/org/springframework/samples/petclinic/web/VetController.java

@@ -41,7 +41,7 @@ public class VetController {
         this.clinicService = clinicService;
     }
 
-    @RequestMapping(value="/vets.xml")
+    @RequestMapping(value={"/vets.xml","/vets.html"})
     public String showVetList(Map<String, Object> model) {
         // Here we are returning an object of type 'Vets' rather than a collection of Vet objects 
         // so it is simpler for Object-Xml mapping