Make the cast in Bidirectional stream smaller

[dvtkrlbs]

Since we already define the type we can just use it instead of writing it again

Motivation

The example on the bidirectional streaming is confusing since we are casting the output
of Box::pin() to a type. However it is the same type as the associated type RouteChatStream.
It makes the code more clean and understandable.

Solution

Use the associated type instead of writing the type again.

[LucioFranco]

👍 good idea! Thanks!