Fix magnify example using right postrender events

examples/magnify.html

@@ -3,7 +3,7 @@ layout: example.html
 title: Magnify
 shortdesc: Show a magnified version of imager under the pointer
 docs: >
-  <p>This example makes use of the <code>postcompose</code> event listener to
+  <p>This example makes use of the <code>postrender</code> event listener to
   oversample imagery in a circle around the pointer location.  Listeners for this event have access to the Canvas context and can manipulate image data.</p>
   <p>Move around the map to see the effect.  Use the ↑ up and ↓ down arrow keys to adjust the magnified circle size.</p>
 tags: "magnify, image manipulation"

examples/magnify.js

@@ -48,7 +48,7 @@ container.addEventListener('mouseout', function() {
 });
 
 // after rendering the layer, show an oversampled version around the pointer
-imagery.on('postcompose', function(event) {
+imagery.on('postrender', function(event) {
   if (mousePosition) {
     const context = event.context;
     const pixelRatio = event.frameState.pixelRatio;