fix README

This PR fixes README to synchronize sample codes/names with latest project codes.
Thank you @harikt.
It's true that `modify()` (and `define()`) is the only api in Aura\Di\Config.
To avoid confusing the difference between this sentence and the code bellow, I suggest adding description as follows:

    In the `modifyWebRouter()` method which is called from `modify()` config method