added unset webhook example

examples/unset.php

+<?php
+//Composer Loader
+$loader = require __DIR__.'/vendor/autoload.php';
+
+$API_KEY = 'your_bot_api_key';
+$BOT_NAME = 'namebot';
+try {
+    // create Telegram API object
+    $telegram = new Longman\TelegramBot\Telegram($API_KEY, $BOT_NAME);
+    // set webhook
+    $result = $telegram->unsetWebHook();
+    //Uncomment to use certificate
+    //$result = $telegram->setWebHook($link, $path_certificate);
+    if ($result->isOk()) {
+        echo $result->getDescription();
+    }
+} catch (Longman\TelegramBot\Exception\TelegramException $e) {
+    echo $e;
+}