net: selftests: Fix optstring

The cited commit added a stray colon to the 'v' option. That makes the
option work incorrectly.

ex:
tools/testing/selftests/net# ./fib_nexthops.sh -v
(should enable verbose mode, instead it shows help text due to missing arg)

Fixes: 5feba472 ("selftests: fib_nexthops: Make ping timeout configurable")
Reviewed-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>