teken: color #3 is yellow not brown - use TC_YELLOW as the name

The console escape code standard (ECMA-48) specifies color #3 (escape
code 33) as yellow. A brown console color is an artifact of the VGA
palette, which replaces dim (but not bright) yellow with brown.

Reviewed by: adrian, imp
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34531