Do not always fold anymore

cups-pdf-postprocess.py

@@ -151,7 +151,7 @@ class CPP(QWidget):
     else:
       cmd = self._printer.currentData().copy()
       cmd += [ "-o", self._duplex.currentData() ]
-      if self._stample.isChecked:
+      if self._stample.isChecked():
         cmd += [ "-o", "XRFold=BiFoldStaple" ]
       cmd.append("-")
       pipeline.append(cmd)