Maubot: Add word to listen to in decorator in getting-started tutorial

[tcassaert]

When trying out the getting-started.md page, it didn't work out of the box.

I had to add @command.new("hello_world") instead of just @command.new().

[tulir]

The code is correct, but the doc after it is not, the command is actually !hello-world

[tcassaert]

You were right. It indeed works with the hyphen instead of the underscore. I've adapted my PR.