wasm: update depreciated getElementById

Change-Id: Ibef29f0fd2cb2012a05400a855cb4985f9164d92
Fixes: QTBUG-74601
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

src/plugins/platforms/wasm/wasm_shell.html

@@ -27,9 +27,9 @@
 
     <script type='text/javascript'>
         function init() {
-          var spinner = document.getElementById('qtspinner');
-          var canvas = document.getElementById('qtcanvas');
-          var status = document.getElementById('qtstatus')
+          var spinner = document.querySelector('#qtspinner');
+          var canvas = document.querySelector('#qtcanvas');
+          var status = document.querySelector('#qtstatus')
 
           var qtLoader = QtLoader({
               canvasElements : [canvas],