14_fibonacci: add negative number example to readme

[GPerencine]

Because

The README for the Fibonacci exercise does not mention that passing a negative number should return "OOPS", even though this behavior is tested in fibonacci.spec.js. Adding this example clarifies the exercise requirements.

This PR

• Adds fibonacci(-1); // returns "OOPS" example to the README code block.

Issue

N/A

Additional Information

N/A

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. 01_helloWorld: Update test cases
• 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 any changes that affect the solution of an exercise, I've also updated the solution in the /solutions folder