Fix problem 85 positional encoding example (Issue #495)

[vishesh-banna0]

Summary

This PR fixes the example for Problem 85: Positional Encoding Calculator.

Changes

• Corrected the example output to match the standard sinusoidal positional encoding described in the original Transformer architecture.
• Fixed the ordering of sine and cosine values (sine at even indices and cosine at odd indices).
• Removed the incorrect extra array dimension from the sample output.
• Updated the reasoning to accurately describe how positional encoding is computed.

Verification

The corrected example output was regenerated using the reference implementation of the positional encoding algorithm.

Correct output generated by the implementation:

Current example displayed on the website:

Closes #495