Fix typos across documentation, code, and tests

[Marcofann]

This pull request addresses several typographical issues in documentation, code comments, and test files, improving clarity and readability across the project.

Summary of Changes:

1. Corrected "a address" to "an address" in various files:

   • index.spec.ts: Updated test descriptions for grammatical correctness.
   • RLPReader.sol: Fixed multiple occurrences of "a address" in function comments.
   • Multisend.sol: Updated comment descriptions for encoding transactions.
   • messages.ts: Corrected "a transaction for an event" to ensure proper phrasing.
   • multisend.ts: Improved inline comments for clarity.

2. Files Modified:

   • index.spec.ts
   • RLPReader.sol
   • Multisend.sol
   • messages.ts
   • multisend.ts