Add forgotten valleys mapgen in mapgen name

Missing `valleys` in `settingtypes.txt` and `minetest.conf.example`.

builtin/settingtypes.txt

@@ -800,7 +800,7 @@ liquid_update (Liquid update tick) float 1.0
 
 #    Name of map generator to be used when creating a new world.
 #    Creating a world in the main menu will override this.
-mg_name (Mapgen name) enum v6 v5,v6,v7,flat,fractal,singlenode
+mg_name (Mapgen name) enum v6 v5,v6,v7,flat,valleys,fractal,singlenode
 
 #    Water surface level of the world.
 water_level (Water level) int 1

minetest.conf.example

@@ -986,7 +986,7 @@
 
 #    Name of map generator to be used when creating a new world.
 #    Creating a world in the main menu will override this.
-#    type: enum values: v5, v6, v7, flat, fractal, singlenode
+#    type: enum values: v5, v6, v7, flat, fractal, valleys, singlenode
 # mg_name = v6
 
 #    Water surface level of the world.