Fix `instance Bounded a => Bounded (Down a)`

Thank you for your contribution to GHC!

Please read the checklist below to make sure your contribution fulfills these expectations. Also please answer the following question in your MR description:

Where is the key part of this patch? That is, what should reviewers look at first?

Small change to instance Bounded (Down a) in base.

Please take a few moments to verify that your commits fulfill the following:

• are either individually buildable or squashed
• have commit messages which describe what they do (referring to Notes and tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a Note and cross-reference it from the relevant places.
• add a testcase to the testsuite.
• if your MR affects library interfaces (e.g. changes base) or affects whether GHC will accept user-written code, please add the ~"user facing" label.
• updates the users guide if applicable
• mentions new features in the release notes for the next release

If you have any questions don't hesitate to open your merge request and inquire in a comment. If your patch isn't quite done yet please do add prefix your MR title with WIP:.

/user-facing