Fix PCTG calibration temperatures (#9111)

It looks like start and end values are swapped. Start needs to be greater than end. If you try to hit "Ok" for the default values, it results in this error:

```
Please input valid values:
Start temp: <= 350
End temp: >= 170
Start temp > End temp + 5)
```

src/slic3r/GUI/calib_dlg.cpp

@@ -432,8 +432,8 @@ void Temp_Calibration_Dlg::on_filament_type_changed(wxCommandEvent& event) {
             end = 230;
             break;
 	case tPCTG:
-            start = 240;
+            start = 280;
-            end = 280;
+            end = 240;
             break;
         case tTPU:
             start = 240;