[otbn] Fix typo when generating error messages
[otbn] Fix typo when generating error messages

Here, args.snippets is an argparse.FileType('r') argument. To print
its name, we need args.snippets.name.

Signed-off-by: Rupert Swarbrick <290093e2ad2515ed2f14d749293c8cf8e02ca274@lowrisc.org>

lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan,lowRISC/opentitan