Add missing const qualifier to snippet in QIdentityProxyModel doc.

Task-number: QTBUG-26751
Change-Id: Icb1f25dd015bcc62ec7c4c2b26c897649f1bbabb
Reviewed-by: Michalina Ziemba <michalina.ziemba@nokia.com>
Reviewed-by: Stephen Kelly <stephen.kelly@kdab.com>

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

@@ -48,7 +48,7 @@ class DateFormatProxyModel : public QIdentityProxyModel
     m_formatString = formatString;
   }
 
-  QVariant data(const QModelIndex &index, int role)
+  QVariant data(const QModelIndex &index, int role) const
   {
     if (role != Qt::DisplayRole)
       return QIdentityProxyModel::data(index, role);