vxlan: fix a typo.

Use eXtensible and not eXtensiable in the comment on top.
Signed-off-by: NRami Rosen <ramirose@gmail.com>
Acked-by: NStephen Hemminger <shemminger@vyatta.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>

drivers/net/vxlan.c

 /*
- * VXLAN: Virtual eXtensiable Local Area Network
+ * VXLAN: Virtual eXtensible Local Area Network
  *
  * Copyright (c) 2012 Vyatta Inc.
  *