Exercise spec: Fix typo in 13_input_output_spec.rb

[annarider]

Because

I changed the word unneccessary to unnecessary

This PR

I fixed the typo by changing the word unneccessary to unnecessary.

Issue

I didn't open an issue for a small typo change. I can open an issue if it is required. The contributing guidelines seem to suggest I could open a PR directly for such a small change.

Additional Information

None

Pull Request Requirements

• I have thoroughly read and understand The Odin Project Contributing Guide
• The title of this PR follows the location of change: brief description of change format, e.g. String spec: Update instructions for clarity
• The Because section summarizes the reason for this PR
• The This PR section has a bullet point list describing the changes in this PR
• If this PR addresses an open issue, it is linked in the Issue section
• If this PR includes changes in the spec folder, they are also updated in the corresponding file in the spec_answers folder (with passing tests).