The netnews.md file does not reference how to use the worldwide
network of Netnews, called Usenet. Instead it only focuses on the
Newsgroups from the "tilde." hierarchy, which I believe are not public
to the Usenet.
Also, the term "Netnews" is more generic, and is the official name
similar to Email or to HTML.
Therefore, remove the term "Usenet" from naming.
Signed-off-by: keyboardan <keyboardan@tilde.club>
