Fix typographical errors in Documentation and Comments

[vipocenka]

This pull request addresses minor typographical errors in documentation and comments across three files. These changes improve readability and ensure consistency with proper grammar. The updates include:

1. rsa.rs:

   • Corrected "a X.509" to "an X.509" in the comment for RSA public key encoding.

2. main.rs:

   • Updated "a SHA-256" to "an SHA-256" in the explanation of peer ID usage.

3. protocol.rs:

   • Fixed "a application-layer" to "an application-layer" in the comment about multistream-select protocol messages.

Notes & Open Questions:

• These changes do not affect functionality or introduce any breaking changes.
• Focused solely on improving comment clarity.

Change Checklist:

• I have performed a self-review of my own code.
• I have made corresponding changes to the documentation.
• I have added tests that prove my fix is effective or that my feature works (not applicable here).
• A changelog entry has been made in the appropriate crates (if needed).