veth.4: Add a more direct example

veth.4: Add a more direct example

iproute2 allows you to specify the netns for either side of a veth
interface at creation time.  Add an example of this to veth(4) so
it doesn't sound like you have to move the interfaces in a
separate step.

Verified with commands:
    # ip netns add alpha
    # ip netns add bravo
    # ip link add foo netns alpha type veth peer bar netns bravo
    # ip -n alpha link show
    # ip -n bravo link show

Signed-off-by: Michael Kerrisk <mtk.manpages@gmail.com>