title: add_filter to reorder the single product tabs causing error
body: <p>I see the following error message when i try to add_fliter to reorder my tabs.
  Here is what i am doing in my child-themes' functions.php</p>
  
  <blockquote>
    <p>PHP Warning:  call_user_func() expects parameter 1 to be a valid callback, no array or string given in wp-content\themes\salient\woocommerce\single-product\tabs\tabs.php on line 47</p>
  </blockquote>
  
  <p>Line 47 in tabs.php points to this -</p>
  
  <pre><code>&lt;?php foreach ( $tabs as $key =&gt; $tab ) : ?&gt;
  
  &lt;div class="panel entry-content" id="tab-&lt;?php echo esc_attr( $key ); ?&gt;"&gt;
      &lt;?php call_user_func( $tab['callback'], $key, $tab ); ?&gt;
  &lt;/div&gt;
  &lt;?php endforeach; ?&gt;
  </code></pre>
  
  <p>add filter am trying to use is this -</p>
  
  <pre><code>add_filter( 'woocommerce_product_tabs', 'woo_reorder_tabs', 98 );
  function woo_reorder_tabs( $tabs ) {
  
      $tabs['reviews']['priority'] = 5;           // Reviews first
      $tabs['description']['priority'] = 10;          // Description second
      $tabs['additional_information']['priority'] = 15;   // Additional information third
  
      return $tabs;
  }
  </code></pre>
  
  <p>I am unable to figure what is wrong. Please help indentify the issue and fix.</p>