iproute2: trivial fix of ip link syntax in manpage

iproute2: trivial fix of ip link syntax in manpage

Reported by Ivan Vilata i Balaguer <ivan@selidor.net>
found that the description of the `ip link add` command in the manpage
is outdated regarding the compulsory `link DEVICE` option.
For instance, `ip  link help` says:
    Usage: ip link add [link DEV] [ name ] NAME
     ...
But the manpage still says:
     ip link add link DEVICE [ name ] NAME

(Trying to provide a `link` option e.g. under an LXC container can frustrate
 the creation of dummy devices which don't need an actual device.)

The syntax of the "ip link help" output was fixed in commit
"iproute2: Fix usage and man page for 'ip link'" (a22e92951d).
This updates the manpage to mark "link DEVICE" as an optional
argument there as well.

  http://bugs.debian.org/673171

Signed-off-by: Andreas Henriksson <andreas@fatal.se>