change_mailchimp_text.php

<?php

function my_pmpro_translate_mailchimp($translated_text, $original_text, $domain) {
	
	switch ( $translated_text ) {
            case 'Join our mailing list.' :
                $translated_text = __( 'Your Changed Text Here', 'pmpro-mailchimp' );
                break;
    }
    return $translated_text;
}
add_filter( 'gettext', 'my_pmpro_translate_mailchimp', 10, 3 );