tcp: fix comment typo

Spell "transmissions" properly.

Found by searching for keyword "tranm".

Signed-off-by: Yueh-Shun Li <shamrocklee@posteo.net>
Link: https://lore.kernel.org/r/20230622012627.15050-6-shamrocklee@posteo.net


Signed-off-by: Jakub Kicinski <kuba@kernel.org>