seg6: fix lenghts typo in a comment

Fix a typo:
  lenghts -> length

The typo has been identified using codespell, and the tool currently
does not report any additional issues in comments.

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250629171226.4988-2-andrea.mayer@uniroma2.it


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