Changed returned attribute from "Game" to "game" (#9945)

I noticed the steam component "game" attribute is capitalized. This should be lowercase if I'm not mistaken.

From:
        return {'Game': self._game}
To:
        return {'game': self._game}

Not sure if i'm doing this correctly... apologizes if I'm not!

homeassistant/components/sensor/steam_online.py

@@ -96,7 +96,7 @@ class SteamSensor(Entity):
     @property
     def device_state_attributes(self):
         """Return the state attributes."""
-        return {'Game': self._game}
+        return {'game': self._game}
 
     @property
     def entity_picture(self):