selftests: netfilter: fix header example

nft_flowtable.sh is made for bash not sh.
Also give values which not return "RTNETLINK answers: Invalid argument"
Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>

tools/testing/selftests/netfilter/nft_flowtable.sh

@@ -11,7 +11,7 @@
 # result in fragmentation and/or PMTU discovery.
 #
 # You can check with different Orgininator/Link/Responder MTU eg:
-# sh nft_flowtable.sh -o1000 -l500 -r100
+# nft_flowtable.sh -o8000 -l1500 -r2000
 #