fix typo in script/util.lua

There is a typo in scriptutil.lua, it will produce a bug below. An error happen when I reload the lua script with the "reload.sh". The log is below.

[20130816 21:49:51 src/qluautil.c:65] load file error
[20130816 21:49:51 src/qluautil.c:66] src/qluautil.c:66 lua_call failed
../script/util.lua:13: bad argument #2 to 'format' (string expected, got nil)

After fix this typo, the server can work normally.