ipvlan: Fix text that talks about ip util support

ipvlan was added into 3.19 release and iproute2 added support
for the same in iproute2-3.19 package.

Signed-off-by: Mahesh Bandewar <maheshb@google.com>
CC: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>