fix typo

PP-71

resources/definitions/ultimaker.def.json

@@ -40,7 +40,7 @@
         "cool_fan_speed_max": { "value": "100" },
         "cool_min_layer_time": { "value": 6 },
         "cool_min_layer_time_fan_speed_max": { "value": "cool_min_layer_time + 5" },
-        "cool_min_speed": { "value": "speed_wall_0 * 3 / 4 if lift_head else speed_wall_0 / 5" },
+        "cool_min_speed": { "value": "speed_wall_0 * 3 / 4 if cool_lift_head else speed_wall_0 / 5" },
         "cool_min_temperature": { "value": "material_print_temperature - 15" },
         "gradual_support_infill_step_height": { "value": "4 * layer_height" },
         "gradual_support_infill_steps": { "value": "2 if support_interface_enable else 0" },