Update index.html

Add PHP grammar file for Prism.js

Author: jhildenbiddle

docs/index.html

@@ -98,6 +98,7 @@
   <script src="//cdn.jsdelivr.net/npm/prismjs/components/prism-bash.min.js"></script>
   <script src="//cdn.jsdelivr.net/npm/prismjs/components/prism-markdown.min.js"></script>
   <script src="//cdn.jsdelivr.net/npm/prismjs/components/prism-nginx.min.js"></script>
+  <script src="//cdn.jsdelivr.net/npm/prismjs/components/prism-php.min.js"></script>
   <script>
   ((window.gitter = {}).chat = {}).options = {
     room: 'docsifyjs/Lobby'