Use common Strings for Z-Wave config flow (#41161)

* Use common Strings for Z-Wave config flow

* Update strings.json

* Update config_flow.py

homeassistant/components/zwave/config_flow.py

@@ -31,7 +31,7 @@ class ZwaveFlowHandler(config_entries.ConfigFlow):
     async def async_step_user(self, user_input=None):
         """Handle a flow start."""
         if self._async_current_entries():
-            return self.async_abort(reason="one_instance_only")
+            return self.async_abort(reason="single_instance_allowed")
 
         errors = {}