out-of-date reference to 0080 used as Runeerror.  use fffd instead.

Reference: /n/sources/patch/applied/utf-runeerror
Date: Sat Apr 29 17:58:55 CES 2006
Signed-off-by: quanstro@quanstro.net

--- /sys/man/6/utf	Sat Apr 29 17:57:09 2006
+++ /sys/man/6/utf	Sat Apr 29 17:57:04 2006
@@ -83,7 +83,7 @@
 .PP
 In the inverse mapping,
 any sequence except those described above
-is incorrect and is converted to rune hexadecimal 0080.
+is incorrect and is converted to rune hexadecimal FFFD.
 .SH FILES
 .TF "/lib/unicode "
 .TP