Fix documentation typo

Author: rstoyanchev

src/reference/docbook/mvc.xml

@@ -2418,7 +2418,7 @@ public String myHandleMethod(WebRequest webRequest, Model model) {
 public Callable<String> processUpload(final MultipartFile file) {
 
   return new Callable<String>() {
-    public Object call() throws Exception {
+    public String call() throws Exception {
       // ...
       return "someView";
     }