netfilter: nft_bitwise: correct uapi header comment.

The comment documenting how bitwise expressions work includes a table
which summarizes the mask and xor arguments combined to express the
supported boolean operations.  However, the row for OR:

 mask    xor
 0       x

is incorrect.

  dreg = (sreg & 0) ^ x

is not equivalent to:

  dreg = sreg | x

What the code actually does is:

  dreg = (sreg & ~x) ^ x

Update the documentation to match.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>