Fix typos in documentation and code comments

[Marcofann]

This pull request corrects various grammatical and typographical errors across multiple README files and Solidity contracts. These changes aim to improve the clarity and professionalism of the project documentation and code comments.

Summary of Changes:

1. README Files:

   • Updated phrases such as "a arbitrage" to "an arbitrage" for grammatical accuracy.
   • Fixed "a example" to "an example" in the EIP-712 explanation.
   • Improved readability and fixed grammar in the DAO/Mirror README.

2. Solidity Contracts:

   • In Vault.sol and VaultWithIndirection.sol:
     • Corrected "a element" to "an element" in multiple comments.