if_stf: minor man page improvements

- fix typo (router -> routers)
- Remove 6bone reference, because that was phased out in 2006.

Suggested by:	Pau Amma <pauamma@gundo.com>
Sponsored by:	Rubicon Communications, LLC ("Netgate")
Differential Revision:	https://reviews.freebsd.org/D33046

share/man/man4/stf.4

@@ -57,9 +57,9 @@ For ordinary nodes in a 6to4 or 6RD site, you do not need
 interface.
 The
 .Nm
-interface is necessary for site border router
+interface is necessary for site border routers
 (called
-.Dq 6to4 router
+.Dq 6to4 routers
 or
 .Dq 6rd Customer Edge (CE)
 in the specification).
@@ -306,8 +306,6 @@ has a WAN address of 192.0.2.2 and the full IPv4 address is embedded in the
 .Xr gif 4 ,
 .Xr inet 4 ,
 .Xr inet6 4
-.Pp
-.Pa http://www.ipv6day.org/action.php?n=En.IPv6day
 .Rs
 .%A Brian Carpenter
 .%A Keith Moore