Update wheelDelta example in README.md (d3#264)

Updated the wheelDelta() example in order to reflect changes made in commit 4d2a5f1.

README.md

@@ -198,7 +198,7 @@ If *delta* is specified, sets the wheel delta function to the specified function
 
 ```js
 function wheelDelta(event) {
-  return -event.deltaY * (event.deltaMode === 1 ? 0.05 : event.deltaMode ? 1 : 0.002);
+  return -event.deltaY * (event.deltaMode === 1 ? 0.05 : event.deltaMode ? 1 : 0.002) * (event.ctrlKey ? 10 : 1);
 }
 ```