gui: clarify needed input for "add sec node" dialog

Change-Id: Ia34eef4df50d545fa779979dd25a239803af0a8e
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/29528
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/gui/mainwindow.py

@@ -140,7 +140,7 @@ class MainWindow(QMainWindow):
     @pyqtSlot()
     def on_actionAdd_SEC_node_triggered(self):
         host, ok = QInputDialog.getText(self, 'Add SEC node',
-                                        'Enter SEC node hostname:')
+                                        'Enter SEC node URI (or just hostname:port):')
 
         if not ok:
             return