Documentation: serial-console: Fix literal block marker

Make rendered text readable by fixing literal block marker, changing
":" to "::".

Signed-off-by: Andrei Emeltchenko <andrei.emeltchenko@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20230825091626.354352-1-Andrei.Emeltchenko.news@gmail.com