Update const.py added missing millimeter (#6834)

* Update const.py added missing millimeter

Added missing millimeter

* Fixed UNIT_MILLIMETER in multiple locations.

esphome/components/mopeka_std_check/sensor.py

@@ -12,6 +12,7 @@ from esphome.const import (
     CONF_TEMPERATURE,
     DEVICE_CLASS_TEMPERATURE,
     UNIT_CELSIUS,
+    UNIT_MILLIMETER,
     STATE_CLASS_MEASUREMENT,
     CONF_BATTERY_LEVEL,
     DEVICE_CLASS_BATTERY,
@@ -26,8 +27,6 @@ ICON_PROPANE_TANK = "mdi:propane-tank"
 
 TANK_TYPE_CUSTOM = "CUSTOM"
 
-UNIT_MILLIMETER = "mm"
-
 
 def small_distance(value):
     """small_distance is stored in mm"""