update "Credits"

components/wifi-manager/index.html

@@ -381,6 +381,7 @@
                             <li>SpinKit, &copy;  2015, Tobias Ahlin. Licensed under the MIT License.</li>
                             <li>jQuery, The jQuery Foundation. Licensed under the MIT License.</li>
                             <li>cJSON, &copy; 2009-2017, Dave Gamble and cJSON contributors. Licensed under the MIT License.</li>
+							<li>esp32-rotary-encoder, &copy; 2011-2019, David Antliff and Ben Buxton. Licensed under the GPL License.</li>
                         </ul>
                     </div>
 	                <h2>Show NVS Editor</h2>