Fix typos across multiple files

[Dimitrolito]

This pull request resolves typographical issues across various documentation files to enhance clarity and consistency:

• SECURITY.md:

  • Corrected "block chain" to "blockchain" and refined phrasing for improved readability.

• README.md:

  • Adjusted indefinite articles from "a" to "an" before words beginning with vowel sounds.

• 0003_token_bridge.md:

  • Corrected improper use of "a" to "an" where necessary.

• 0004_message_publishing.md:

  • Fixed similar article-related typos for consistent style.

These updates improve the overall quality of the project's documentation.