doc: nft.8 more spelling fixes

doc: nft.8 more spelling fixes

I ran the following command:

ispell -p ./ispell_nft -H nft.xml

to create the local dictionary ispell_nft.
ispell_nft contains almost every special word in nft.xml.
The idea is that anyone can run ispell the same way and only have to accept:

 - alpha strings in hexadecimal numbers
 - "FIXME" : that has to be fixed eventually
 - "differv" : I don't know what that is or whether it's correct

You need to use the English (i.e. American) dictionary, and you want the screen
to be about 100 chars wide (at least).

The patch enforces consistent capitalisation of words, e.g. IPv4 is always that
way but ipv4_addr stays as before. The existing dictionary suggested capital
Ethernet so that is in there too.

Signed-off-by: Duncan Roe <duncan_roe@optusnet.com.au>
Signed-off-by: Florian Westphal <fw@strlen.de>