feat: Improve sleeper train icon

Author: MoritzWeber0

layouts/partials/train-category.html

@@ -5,7 +5,7 @@
         {{ $iconMapping := dict
           "highspeed" "train"
           "regional" "directions_subway"
-          "sleeper" "airline_seat_flat"
+          "sleeper" "hotel"
           "funicular" "funicular"
           "bus" "directions_bus"
           "tram" "tram"