flatten-array: fix nesting

[ErikSchierboom]

I erronously used c() for nesting, but that won't actually create nesting. Switched to list() Sorry!

[github-actions]

This PR touches files which potentially affect the outcome of the tests of an exercise. This will cause all students' solutions to affected exercises to be re-tested.

If this PR does not affect the result of the test (or, for example, adds an edge case that is not worth rerunning all tests for), please add the following to the merge-commit message which will stops student's tests from re-running. Please copy-paste to avoid typos.

[no important files changed]

For more information, refer to the documentation. If you are unsure whether to add the message or not, please ping @exercism/maintainers-admin in a comment. Thank you!

[colinleach]

Ouch, it's almost as though the reviewer wasn't doing his job...

[colinleach]

I'm assuming no students have done this new exercise, so there's nothing to re-test.