[Groff] small bug in tbl

[Alejandro Lopez-Valencia]

Reading tbl(1), it states that both "_" and "-" are recognized as line 
generators. Yet, it is not so...

This works:

.TS
center;
cB cB
l l.
_
Number  Name
_
1       ichi
2       ni
3       san
4       shi
5       go
_
.TE

But this doesn't:

.TS
center;
cB cB
l l.
-
Number  Name
-
1       ichi
2       ni
3       san
4       shi
5       go
-
.TE

And, btw, in traditional typesetting, those three lines would have 
different widths. Could that be added easily?