Create Joomla extension to show Hello World on every page

<?xml version="1.0" encoding="utf-8"?>
<extension version="3.1" type="plugin" group="system" method="upgrade">
    <!-- This name tag is a user friendly name and you can name it the way you want. -->
    <name>PramukhIME</name>
    <author>Vishal Monpara</author>
    <creationDate>October, 2017</creationDate>
    <copyright>Copyright (C) 2017 Vishal Monpara. All rights reserved.</copyright>
    <license>see license.html</license>
    <authorEmail>myemailaddress@something.com</authorEmail>
    <authorUrl>www.vishalon.net</authorUrl>
    <version>1.0.0</version>
    <files>
        <!-- plugin tag value must be plugin name and the php file must be the pluginname.php -->
        <filename plugin="pramukhime">pramukhime.php</filename>
    </files>
</extension>

<?php
/**
 * @package     Joomla.Plugin
 * @subpackage  System.PramukhIME
 *
 * @copyright   Copyright (C) 2017 Vishal Monpara. All rights reserved.
 * @license     see license.html
 */
 
defined('_JEXEC') or die;
 
class PlgSystemPramukhIME extends JPlugin
{
    protected $app;
 
    public function onAfterDispatch()
    {
        // Uncomment following line to add your own css/javascript file in head
        //JHtml::_('stylesheet', 'media/path/to/pramukhindic.css', array('version' => 'auto', 'relative' => false));
        //JHtml::_('script', 'media/path/to/pramukhime.js', array('version' => 'auto', 'relative' => false));
    }
     
    public function onAfterRender()
    {
        // Get page html which includes full page
        $body = $this->app->getBody();
        $content = "<h1>Hello World</h1>";
        // locate </body> tag and insert our own content right before </body>
        $body = str_replace('</body>', $content . '</body>', $body );
        // set the body
        $this->app->setBody($body);
    }
}