Add parameter for function call in QIdentityProxyModel

This change ensures that the correct model index is passed to the sourceModel()->data() function when retrieving the DateRole value.

Pick-to: 6.8 6.5
Fixes: QTBUG-70792
Change-Id: I66c9f2404533625f05dd51db2d9263287db6ee91
Reviewed-by: Giuseppe D'Angelo <giuseppe.dangelo@kdab.com>

src/corelib/doc/snippets/code/src_gui_itemviews_qidentityproxymodel.cpp

@@ -16,7 +16,8 @@ class DateFormatProxyModel : public QIdentityProxyModel
     if (role != Qt::DisplayRole)
       return QIdentityProxyModel::data(index, role);
 
-    const QDateTime dateTime = sourceModel()->data(SourceClass::DateRole).toDateTime();
+    const QModelIndex sourceIndex = mapToSource(index);
+    const QDateTime dateTime = sourceModel()->data(sourceIndex, SourceClass::DateRole).toDateTime();
     return dateTime.toString(m_formatString);
   }