Doc: fix wrong link to the example snippet in QApplication class

Change-Id: Ib9a1b107dd8d2cd4bf541c3edf19a602fde6356f
Reviewed-by: Cristian Maureira-Fredes <cristian.maureira-fredes@qt.io>
Reviewed-by: Martin Smith <martin.smith@qt.io>
Reviewed-by: Paul Wicking <paul.wicking@qt.io>

src/widgets/kernel/qapplication.cpp

@@ -2868,7 +2868,7 @@ void QApplication::setStartDragDistance(int l)
     and the current position (e.g. in the mouse move event) is \c currentPos,
     you can find out if a drag should be started with code like this:
 
-    \snippet code/src_gui_kernel_qapplication.cpp 6
+    \snippet code/src_gui_kernel_qapplication.cpp 7
 
     Qt uses this value internally, e.g. in QFileDialog.