Update to API.md

docs/API.md

@@ -335,12 +335,12 @@ code-splitting.
 ##### `children`
 Routes can be nested, `this.props.children` will contain the element created from the child route component. Please refer to the [Route Configuration](/docs/guides/RouteConfiguration.md) since this is a very critical part of the router's design.
 
-##### `onEnter(nextState, replace, callback?)`
+##### `onEnter(nextState, replace, [callback])`
 Called when a route is about to be entered. It provides the next router state and a function to redirect to another path. `this` will be the route instance that triggered the hook.
 
 If `callback` is listed as a 3rd argument, this hook will run asynchronously, and the transition will block until `callback` is called.
 
-##### `onChange(prevState, nextState, replace, callback?)`
+##### `onChange(prevState, nextState, replace, [callback])`
 Called on routes when the location changes, but the route itself neither enters or leaves. For example, this will be called when a route's children change, or when the location query changes. It provides the previous router state, the next router state, and a function to redirect to another path. `this` will be the route instance that triggered the hook.
 
 If `callback` is listed as a 4th argument, this hook will run asynchronously, and the transition will block until `callback` is called.