docs: improve routed IP address within VPP L3fwd user guide

Change routed IP address from 10.0.0.0/32 to 10.0.0.1/32 for
consistency with "show ip fib" output and better understanding.

Signed-off-by: Liangxing Wang <liangxing.wang@arm.com>
Reviewed-by: Nathan Brown <nathan.brown@arm.com>
Change-Id: I7a3b646629e0af1455120612a500eb45e247c551