selftests: seg6: fix instaces typo in comments

Fix a typo:
  instaces -> instances

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

Signed-off-by: Andrea Mayer <andrea.mayer@uniroma2.it>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20250629171226.4988-3-andrea.mayer@uniroma2.it


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