Fix the button placement.

examples/vr_simplest/index.html

@@ -73,7 +73,7 @@
 
 				// HMDs require the call to start presenting to occur due to a user input event, so make a button to trigger that
 				let enterVRButton = document.createElement('button')
-				target.appendChild(enterVRButton)
+				document.getElementById('target').appendChild(enterVRButton)
 				enterVRButton.innerHTML = 'enter vr'
 				enterVRButton.addEventListener('click', ev => {
 					window.pageApp.startPresenting()