Send correct command to pyrfxtrx

Although it seems to work with send_on, it throws an logged error. Using correct command against pyrfxtrx removes this error.

homeassistant/components/light/rfxtrx.py

@@ -149,7 +149,7 @@ class RfxtrxLight(Light):
             self._brightness = ((brightness + 4) * 100 // 255 - 1)
 
         if hasattr(self, '_event') and self._event:
-            self._event.device.send_on(rfxtrx.RFXOBJECT.transport,
+            self._event.device.send_dim(rfxtrx.RFXOBJECT.transport,
                                        self._brightness)
 
         self._brightness = (self._brightness * 255 // 100)