Fix typo in default settings

This typo causes the plugin to fail for new installations. If you
have already configured it, these values are not used.

Fixes #12

Author: ali1234

filament_scale_enhanced/__init__.py

@@ -31,7 +31,7 @@ def get_template_configs():
     def get_settings_defaults():
         return dict(
             tare=8430152,
-            refernce_unit=-411,
+            reference_unit=-411,
             spool_weight=200,
             clockpin=21,
             datapin=20,