Fix documentation formatting and link issues

[donatik27]

1. bp256/README.md:
   Old: Rust **1.81* or higher.
   New: Rust **1.81** or higher.
   Reason: Fixes incorrect Markdown bold text formatting by adding the missing closing asterisk. This ensures proper rendering of the minimum Rust version requirement.

2. primeorder/README.md:
   Old: [p521]: https://github.com/RustCrypto/elliptic-curves/tree/master/p384
   New: [p521]: https://github.com/RustCrypto/elliptic-curves/tree/master/p521
   Reason: Corrects the incorrect URL path for the p521 documentation link. The original link incorrectly pointed to p384 instead of p521, which could confuse users looking for p521 documentation.

Before:

After:

These changes improve documentation accuracy and ensure proper navigation within the project documentation.

Testing:

• Verified Markdown formatting renders correctly
• Confirmed the p521 link points to the correct directory