You can change it using small code in the Code Snippets plugin or directly in the functions.php file of your child WordPress theme:

function custom_reset_password_email_subject($subject, $user_login, $user_data) {
    // Change the subject to your custom subject
    $subject = 'Your Custom Reset Password Subject';
    return $subject;
}
add_filter('retrieve_password_title', 'custom_reset_password_email_subject', 10, 3);

The doc: https://developer.wordpress.org/reference/hooks/retrieve_password_title/