Changed to 'set' instead of 'add', added thumbnail.

Just a real simple mod i made that increases the distance roads are spawned by 2. I make no promises that theres not any weird edge case bugs, but with this simple a change there shouldn't be.Edit: Hopefully i did the 'update existing mod' stuff correctly. This update is to increase the 'max road path length' variable as well as the 'max road tile distance' one.