display: Use the active extruder in the default 16x4 screen layout

Change the default layout to show the active extruder (not always the
first extruder).

Signed-off-by: Kevin O'Connor <kevin@koconnor.net>

klippy/extras/display/display.cfg

@@ -73,7 +73,9 @@ text:
 
 [display_data _default_16x4 extruder]
 position: 0, 0
-text: { render("_heater_temperature", param_heater_name="extruder") }
+text:
+  {% set active_extruder = printer.toolhead.extruder %}
+  { render("_heater_temperature", param_heater_name=active_extruder) }
 
 [display_data _default_16x4 fan]
 position: 0, 10