stairs: typofix for dark_grey wool

stairs.lua

 for _,name in ipairs({"red", "green", "white", "black", "blue", "brown", "cyan",
-                    "grey", "dark_green", "woold:dark_grey", "magenta", "orange", "pink",
+                    "grey", "dark_green", "dark_grey", "magenta", "orange", "pink",
                     "red", "violet", "yellow"}) do
     local nodename = "wool:"..name
     local tmp = minetest.registered_nodes[nodename]