trafgen: fix checksumming in trafgen example config

Took quite a while to git bisect the cause for the wrong TCP checksum
in the -e example. It turned out that commit bf43e1993c7037 ("trafgen:
lexer: return original string if no shellcode") "broke" it, since
before that commit the TCP checksum from -e example was correct and
afterwards not anymore. Well, it didn't break it. What was happening
here is that with this fix above, the packet got 1 byte longer since
the first character of the example string is not omitted anymore,
therefore the checksum got wrong. Fix this by fixing the IP total
length of the packet in the -e and man page example. The UDP example
from the man page still works well if csumudp() is used, so not
affected of this.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>