fix(svm): make token_mint_address not optional in the function definition docs

[aspnxdd]

In the FunctionImplementation for GetAssociatedTokenAccount, the 2nd arg (args.get(1)) is required, but in the function definition it says it's optional. It is indeed not optional.

       let token_mint_address = SvmValue::to_pubkey(args.get(1).unwrap()).map_err(|e| {
            to_diag(
                fn_spec,
                format!("invalid token mint address for getting associated token account: {e}"),
            )
        })?;

This PR changes the optional value in the function definition.