Improve description of `tibber.get_prices` action (#139863)

Replace with the description from the online docs which add the information that a price level is included.

This also makes it consistent with the standard descriptive style in Home Assistant.

homeassistant/components/tibber/strings.json

@@ -87,7 +87,7 @@
   "services": {
     "get_prices": {
       "name": "Get energy prices",
-      "description": "Get hourly energy prices from Tibber",
+      "description": "Fetches hourly energy prices including price level.",
       "fields": {
         "start": {
           "name": "Start",