Fix whitespace errors to silence luacheck.

Currently all minetest_game PR's are failing travis since the
upstream luacheck now also warns about whitespace issues, and
there are a few of those in the code. This fixes all of them
so we can yet again rely on luacheck.

mods/doors/init.lua

@@ -519,7 +519,7 @@ function doors.register_trapdoor(name, def)
 	if not name:find(":") then
 		name = "doors:" .. name
 	end
-	
+
 	local name_closed = name
 	local name_opened = name.."_open"