add thread option to roundup-server.1 man page.

Author: rouilj

share/man/man1/roundup-server.1

@@ -20,7 +20,7 @@ Run the server in the background and write the server's PID to the
 file indicated by PIDfile. The -l (or -L) option \fBmust\fP be
 specified if -d is used.
 .TP
-\fB-t\fP \fBfork|debug|none\fP
+\fB-t\fP \fBfork|thread|debug|none\fP
 Control multi-process mode. Default fork.
 .TP
 \fB-l\fP \fIfile\fP