QFutureWatcher: Refer to convenience method resultAt in documentation

The future does not actually have a result() member function that takes an int.
The correct function is resultAt(). But that is also available directly in the
QFutureWatcher, so refer to that instead of advising to get to the future.

Change-Id: I53d267b4b48b1171bf611e11130b9dacabc059a4
Reviewed-by: Martin Smith <martin.smith@qt.io>

src/corelib/thread/qfuturewatcher.cpp

@@ -576,7 +576,7 @@ void QFutureWatcherBasePrivate::sendCallOutEvent(QFutureCallOutEvent *event)
     This signal is emitted when the watched future reports a ready result at
     \a index. If the future reports multiple results, the index will indicate
     which one it is. Results can be reported out-of-order. To get the result,
-    call future().result(index);
+    call resultAt(index);
 */
 
 /*!