Add samples for Invoice Template API

sample/invoice-templates/UpdateInvoiceTemplate.php

@@ -0,0 +1,33 @@
+<?php
+
+// # Update Invoice Sample
+// This sample code demonstrate how you can update
+// an invoice.
+
+/** @var Template $template */
+$template = require 'GetInvoiceTemplate.php';
+use PayPal\Api\Template;
+
+
+// ### Update Invoice
+$template->setUnitOfMeasure("QUANTITY");
+
+// ### NOTE: These are the work-around added to the
+// sample, to get past the bug in PayPal APIs.
+$template->setCustom(null);
+
+// For Sample Purposes Only.
+$request = clone $template;
+try {
+    // ### Update Invoice Template
+    // Update an invoice by calling the invoice->update() method
+    // with a valid ApiContext (See bootstrap.php for more on `ApiContext`)
+    $template->update($apiContext);
+} catch (Exception $ex) {
+    // NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
+    ResultPrinter::printError("Invoice Template Updated", "Invoice", null, $request, $ex);
+    exit(1);
+}
+
+// NOTE: PLEASE DO NOT USE RESULTPRINTER CLASS IN YOUR ORIGINAL CODE. FOR SAMPLE ONLY
+ ResultPrinter::printResult("Invoice Template Updated", "Invoice", $template->getTemplateId(), $request, $template);