Hi @kragil,
That guide is outdated, as you can see here the match_signal mentioned in the error you get was added after that guide was written.
For you to advance, adding the match_signal function after the others match functions should be enough. An example of a snippet can be found here in our documentation.
Something along these lines:
function M.match_signal(context, dispatcher, tick, state, data)
return state, "signal received: " .. data
end
To clarify the other replies, the matchmaker_matched is registered here but I believe you have done that already because the match_signal error happens within the function match_create which is called in the function register at line 45.
For the sake of curiosity, more details about the hook concept can be found here.