Stairs: Fix smooth lighting of stair mesh

mods/stairs/models/stairs_stair.obj

@@ -55,7 +55,8 @@ usemtl None
 s off
 f 13/11/3 14/12/3 15/13/3
 f 15/13/3 18/14/3 17/15/3
-f 14/12/3 16/16/3 18/14/3
+f 14/12/3 16/16/3 15/13/3
+f 16/16/3 18/14/3 15/13/3
 o stairs_left
 v 0.500000 0.000000 0.000000
 v 0.500000 -0.500000 -0.500000
@@ -75,7 +76,8 @@ usemtl None
 s off
 f 19/17/4 20/18/4 21/19/4
 f 19/17/4 23/20/4 24/21/4
-f 20/18/4 24/21/4 22/22/4
+f 20/18/4 19/17/4 22/22/4
+f 19/17/4 24/21/4 22/22/4
 o stairs_back
 v -0.500000 -0.500000 0.500000
 v 0.500000 -0.500000 0.500000