Fix spelling mistakes

[NikEmman]

Because

Spelling issues on files /spec/16a and /spec_answers/16a

This PR

Fixes the spelling

• word existance
• word occuring

Issue

Additional Information

Pull Request Requirements

• [x] I have thoroughly read and understand The Odin Project Contributing Guide
• [x] The title of this PR follows the location of change: brief description of change format, e.g. String spec: Update instructions for clarity
• [x] The Because section summarizes the reason for this PR
• [x] The This PR section has a bullet point list describing the changes in this PR
• [x] If this PR addresses an open issue, it is linked in the Issue section
• [x] 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).