D8: redirect after user update

demo.php

<?php

/**
 * Implements hook_form_FORM_ID_alter().
 */
function mymodule_form_user_form_alter(&$form, FormStateInterface $form_state, $form_id) {
  if ($form_id === 'user_form') {
    $form['actions']['submit']['#submit'][] = '__mymodule_form_user_submit';
  }
}

/**
 * Redirect to /user
 */
function __mymodule_form_user_submit(&$form, FormStateInterface $form_state) {
  $form_state->setRedirect('user.page');
}