Fix documentation of MotdPhrase length (#254)

Fix documentation of MotdPhrase length (#254)

The max length is actually 126 (< 127), since the check errors out if
length >= 127. See
<https://github.com/ngircd/ngircd/blob/master/src/ngircd/conf.c#L1487>.

I didn't look through the history to see when the change happened. I
just happened to find during a migration that my 140 character MOTD
didn't work.

Update sample configuration file as well as the man page.