Minor: add comment about index shift: 4 -> 5

UpdatePTUDevCtrl/update.cpp

@@ -651,6 +651,7 @@ bool Update::checkDownloadedJsonVersions(QStringList const& jsonFileNames) {
                 bool ok = false;
                 int n = captured.toInt(&ok);
                 if (ok) {
+                    // note: the '5' is correct
                     jsonNr = n + 5;
                 }
             }