Update SPEED_TO_MPH constant to correct value

User @Glenwing reported in #988 that the speed constant was wrong; should be `31.06f` (not `32.06f`).

Fixes #988

TLM/TLM/Constants.cs

@@ -20,7 +20,7 @@ public static class Constants {
         /// Conversion rate from MPH to game speed (also exists in TrafficManager.API.Constants)
         /// </summary>
         [UsedImplicitly]
-        public const float SPEED_TO_MPH = 32.06f; // 50 km/h converted to mph
+        public const float SPEED_TO_MPH = 31.06f; // 50 km/h converted to mph
 
         /// <summary>
         /// Screen pixel size for overlay signs, such as one-per-segment speed limits.
@@ -50,4 +50,4 @@ public static class Constants {
 
         public static INotifier Notifier => TrafficManager.Notifier.Instance;
     }
-}
+}