vtep: Use unicast MAC as an example for a unicast destination in README.

Until now, the example for a unicast MAC has been a multicast MAC.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Justin Pettit <jpettit@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>