Doc: Fix typo in snippet

Resolves "reference to non-static member function must be called" error.

Fixes: QTBUG-72403
Change-Id: Iebd865ff553736df43548b72b45ed3f4711fffc1
Reviewed-by: Jesus Fernandez <Jesus.Fernandez@qt.io>

src/gui/doc/snippets/code/src_gui_vulkan_qvulkaninstance.cpp

@@ -102,7 +102,7 @@
         }
 
         bool event(QEvent *e) {
-            if (e->type == QEvent::UpdateRequest)
+            if (e->type() == QEvent::UpdateRequest)
                 render();
             return QWindow::event(e);
         }