Change password protected text

Hi everyone,

I tryed to change the text of the password protected page, with no results.

These is the code I used:

<?php
function my_password_form() {
    global $post;
    $label = 'pwbox-'.( empty( $post->ID ) ? rand() : $post->ID );
    $o = '<form action="' . esc_url( site_url( 'wp-login.php?action=postpass', 'login_post' ) ) . '" method="post">
    ' . __( "Prova" ) . '
    <label for="' . $label . '">' . __( "Password:" ) . ' </label><input name="post_password" id="' . $label . '" type="password" size="20" maxlength="20" /><input type="submit" name="Submit" value="' . esc_attr__( "Submit" ) . '" />
    </form>
    ';
    return $o;
}
add_filter( 'the_password_form', 'my_password_form' );
?>

Why does it not work? It should show the text “Prova”, but it keeps showing the original one.