Http API docs: Make it more clear which part of TransactionSignature must be Base64 encoded

[Executus]

In the "Submit a transaction" request section, payload_signatures and envelope_signatures have the description "A list of Base64 encoded signatures". I'd suggest moving "Base64 encoded" to the description of "signature" inside the array of TransactionSignatures, as this is the property that actually needs to be Base64 encoded.