example: use copy_bidirectional in proxy.rs

[kiron1]

Motivation

The examples/proxy.rs was created before copy_bidirectional #3572 was added to tokio. In #2590 the proxy example was updated to close pending connections. By reusing the copy_bidirectional the example can be simplified.

Solution

Reuse copy_bidirectional in example.

[Darksonn]

Thanks.