Update Socket.IO URL in readme.

README.md

@@ -96,7 +96,7 @@ Reveal.initialize({
 		{ src: 'lib/js/showdown.js', condition: function() { return !!document.querySelector( '[data-markdown]' ); } },
 		// Speaker notes support
 		{ src: 'plugin/speakernotes/client.js', async: true, condition: function() { return window.location.host === 'localhost:1947'; } },
-		{ src: 'socket.io/socket.io.js', async: true, condition: function() { return window.location.host === 'localhost:1947'; } },
+		{ src: '/socket.io/socket.io.js', async: true, condition: function() { return window.location.host === 'localhost:1947'; } },
 	]
 });
 ```