Docs: Update default value for zoom.filter

Commit ab4bc64 changed the default value for `zoom.filter`. This was not reflected in the documentation. This PR resolves that.

README.md

@@ -162,7 +162,7 @@ If *filter* is specified, sets the filter to the specified function and returns
 
 ```js
 function filter(event) {
-  return !event.ctrlKey && !event.button;
+  return (!event.ctrlKey || event.type === 'wheel') && !event.button;
 }
 ```