selftests/net/pmtu.sh: Fix typo in error message

The word 'expected' was spelled as 'exepcted'.
Fixed the typo in this patch.

Signed-off-by: Abhash Jha <abhashkumarjha123@gmail.com>
Reviewed-by: Simon Horman <horms@kernel.org>
Link: https://patch.msgid.link/20240814173121.33590-1-abhashkumarjha123@gmail.com


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