Fix entity_id to follow HA conventions (#263)

Seen since HA 2026.2.0:

```
Detected that custom integration 'spothinta' sets an invalid entity ID: 'sensor.spothinta_FI_energy_min_price_tomorrow'. In most cases, entities should not set entity_id, but if they do, it should be a valid entity ID.. This will stop working in Home Assistant 2027.2.0, please create a bug report at https://github.com/slovdahl/home-assistant-spothinta/issues
```

Author: slovdahl

custom_components/spothinta/sensor.py

@@ -170,7 +170,7 @@ def __init__(
         """Initialize Spot-Hinta.fi sensor."""
         super().__init__(coordinator=coordinator)
         self.entity_description = description
-        self.entity_id = f"{SENSOR_DOMAIN}.{DOMAIN}_{coordinator.region.name}_{description.service_type}_{description.key}"
+        self.entity_id = f"{SENSOR_DOMAIN}.{DOMAIN}_{coordinator.region.name.lower()}_{description.service_type}_{description.key}"
         self._attr_unique_id = f"{coordinator.config_entry.entry_id}_{description.service_type}_{description.key}"
         self._attr_device_info = DeviceInfo(
             entry_type=DeviceEntryType.SERVICE,