banner: Correct history.

Banner appeared in the 6th edition of AT&T Research unit. It was
subsequently on all the Berkeley tapes, as well as PWB, System III and
System V. The PWB/AT&T and BSD banner programs were different, and the
current FreeBSD banner program shares many elements of the 3BSD one,
though the font has changed.

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usr.bin/banner/banner.6

@@ -28,7 +28,7 @@
 .\"	From: @(#)banner.6	8.2 (Berkeley) 4/29/95
 .\" $FreeBSD$
 .\"
-.Dd May 26, 2020
+.Dd June 21, 2021
 .Dt BANNER 6
 .Os
 .Sh NAME
@@ -64,7 +64,7 @@ suitable for a narrow terminal.
 The
 .Nm
 utility first appeared in
-.Bx 4.4 .
+.At 6 .
 .Sh AUTHORS
 .An Mark Horton
 .Sh BUGS