docs: tproxy: fix formatting for nft code block

The nft command snippet for redirecting traffic isn't formatted
in a literal code block like the rest of snippets.
Fix the formatting inconsistency.

Signed-off-by: Chen Linxuan <chenlinxuan@uniontech.com>
Reviewed-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>