pktgen: adjust spacing in proc file interface output

Too many spaces were introduced in commit 63adc6fb ("pktgen: cleanup
checkpatch warnings"), thus misaligning "src_min:" to other columns.

Fixes: 63adc6fb ("pktgen: cleanup checkpatch warnings")
Signed-off-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>