Improvement of type spec for balance method of bank account exercidse

[serhiy-yevtushenko]

I added {:ok, account_closed} to balance type spec, as this is expected output for this exercise according to test

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Automated comment created by PR Commenter 🤖.

[angelikatyborska]

Thank you! Could you make the same change in the example solution?

[serhiy-yevtushenko]

Done - added to .meta/example.ex (I hope that this is example solution)

[angelikatyborska]

Perfect!