get_cvar_string doesn't work like that.
This is how I've done it in the past. The message displays the scoreboard but is obviously not required. amx_nextmap isn't guaranteed to exist if you were planning to release your plugin.
Code:
...
message_begin(MSG_ALL, SVC_INTERMISSION)
message_end()
set_task(2.0, "change_map")
...
public change_map()
{
new nextmap[32]
get_cvar_string("amx_nextmap", nextmap, 31)
server_cmd("changelevel %s", nextmap)
}