docs: fix minor typos and improve backtick consistency

[Vegapunk-debug]

Fix minor typos and improve documentation consistency

Description

This PR addresses several minor typos and consistency issues in the ky documentation and source code comments to ensure a high-quality, professional appearance and better readability:

• Standardized Terminology: Corrected "Ky-instances" to "Ky instances" in the readme.md and type definitions for better style consistency.
• Improved Syntax Highlighting: Added missing backticks to key code symbols like response and undefined in readme.md code block comments.
• Professional JSDoc Formatting: Added missing backticks to internal error class names (KyError, HTTPError, NetworkError, TimeoutError, and ForceRetryError) in source/utils/type-guards.ts and source/core/Ky.ts comments.

Type of Change

• Documentation update (non-breaking change; fixes typos and formatting)

Checklist

• My changes follow the code style of this project.
• I have performed a self-review of my own changes.
• My changes generate no new warnings.

Thank You!

[Vegapunk-debug]

@sindresorhus @sholladay , could you please review my PR