Remove redundant link to archived Starknet Book

[Giri-Aayush]

The current README lists both the Starknet Documentation and The Starknet Book as separate resources:

• Starknet Documentation - Official Documentation.
• The Starknet Book - In-depth guide.

However, The Starknet Book has been archived, and both links now redirect to the same Starknet Documentation page. This redundancy should be removed to avoid confusion.

Proposed change:
Remove the line for The Starknet Book, keeping only the link to the official Starknet Documentation.

Rationale:
While The Starknet Book may contain relevant material, it is no longer maintained. Directing users to the official, actively maintained Starknet Documentation ensures they access the most current and accurate information. This approach aligns with best practices in documentation management and helps prevent the dissemination of potentially outdated information in a rapidly evolving ecosystem.

Screenshot showing the archive notice:

This change will help keep the resource list up-to-date, prevent users from encountering redundant or potentially outdated information, and ensure they are directed to the most current and authoritative source of Starknet documentation.