docs(guide/directive): fix myDraggable for zoomed page

If you have zoomed into the page in your browser then the screen coordinate system no longer
matches the page coordinate system.  To ensure that dragged elements work correctly when zoomed
we should use pageX/pageY rather than screenX/screenY.

Closes angular#4687

docs/content/guide/directive.ngdoc

@@ -748,15 +748,15 @@ element?
           element.on('mousedown', function(event) {
             // Prevent default dragging of selected content
             event.preventDefault();
-            startX = event.screenX - x;
-            startY = event.screenY - y;
+            startX = event.pageX - x;
+            startY = event.pageY - y;
             $document.on('mousemove', mousemove);
             $document.on('mouseup', mouseup);
           });
 
           function mousemove(event) {
-            y = event.screenY - startY;
-            x = event.screenX - startX;
+            y = event.pageY - startY;
+            x = event.pageX - startX;
             element.css({
               top: y + 'px',
               left:  x + 'px'