Fix maze on LuaJit.

I've just downloaded this game, and the maze mod seems to fail to load on my LuaJit-enabled local server with the following error:

```
09:26:03: ERROR[main]: ...e/diego/src/minetest/bin/../games/OCD/mods/maze/init.lua:363: invalid escape sequence near '"default'
```

I've tested similar code both under standalone Lua 5.1 (works OK), and standalone LuaJit (fails). It may be worth mentioning that Lua 5.2 fails as well.

This patch fixes this error, without affecting the game under plain Lua 5.1.