typo fix for vis.3

[Brooks Davis <brooks%freebsd.org@localhost>]

Kurt Jaeger submitted the following patch to vis.3 which we import into
FreeBSD from NetBSD.

-- Brooks

https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=210013

--- contrib/libc-vis/vis.3	2016-06-04 06:46:24.234726000 +0200
+++ contrib/libc-vis/vis.3	2016-06-04 06:47:13.515516000 +0200
@@ -513,7 +513,7 @@ 
 .Nx 6.0
 and
 .Fx 9.2 .
-Myltibyte character support was added in
+Multibyte character support was added in
 .Nx 7.0
 and
 .Fx 9.2 .