Documentation/tcp: Fix an obvious typo

Yep, my VIM spellchecker is not good enough for typos like this one.

Fixes: 7fe0e38b ("Documentation/tcp: Add TCP-AO documentation")
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Reported-by: Markus Elfring <Markus.Elfring@web.de>
Closes: https://lore.kernel.org/all/2745ab4e-acac-40d4-83bf-37f2600d0c3d@web.de/


Signed-off-by: Dmitry Safonov <dima@arista.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>