Fix documentation comment links in Client class

The links in [these lines](https://github.com/dart-lang/http/blob/eb11c61e7bc14553e3256acafdb4d0096df18c27/lib/src/client.dart#L18-L19) link to the wrong functions. They should instead link to [this one](https://github.com/dart-lang/http/blob/eb11c61e7bc14553e3256acafdb4d0096df18c27/lib/http.dart#L35) for example (the `head` function in this particular example).