Update Links in Documentation

[Pronoss]

1. Link Updates in mod.rs:

   • Replaced outdated links to p256/src/arithmetic/projective.rs with updated links pointing to p256/tests/projective.rs in RustCrypto/elliptic-curves.

2. Typographical Error Fixes:

   • Corrected the usage of "a" to "an" in cmp.rs to adhere to proper grammar rules.
   • Updated "an little-endian" to "a little-endian" in mod.rs to ensure grammatical accuracy.

Checklist

• Targeted PR against the correct branch (master).
• Linked to GitHub issue with discussion or provided an explanation for the updates.
• Verified grammar changes for accuracy.
• Ensured updated links lead to the correct resources.
• Reviewed changes in the "Files changed" section.