Change comment in `contrib/amcheck` regression suite.

I did find a misleading comment in amcheck's regression test suite,
during un-related hacking in Cloudberry[0]. There is a comment which
says `-- Check that sequences are rejected` but those are supported
starting c3b011d

Lets make things not misleading.



[0] https://github.com/apache/cloudberry
-- 
Best regards,
Kirill Reshke