doc/VERSION_LETTERS: correct description of 'n'

This letter does not necessarily mean that services is attached to a
terminal; but rather that it was simply started with -n to not fork
to the background. This is entirely normal when running under some
kind of supervisor e.g. OpenRC's supervise-daemon(8).

doc/VERSION_LETTERS

@@ -22,6 +22,6 @@ d - debug logging is enabled
 e - nick/account registration requires email verification
 j - join_chans is enabled
 l - leave_chans is enabled
-n - services is attached to a terminal
+n - services is not running as a background daemon
 r - operserv raw can be used
 t - changets is enabled (shown as of 2.1)