Show expected client/server terminal output at the end of channels tutorial

[AlfredWilmot]

I really like the approach of documenting the expected terminal output of code-snippets in the tutorial, as this lets the reader sanity-check their own code, which also helps with engagement by encouraging them to run the tutorial code themselves.

Here's an example of this from the spawning section:

website/content/tokio/tutorial/spawning.md

Lines 65 to 88 in ec56f48

	Now, run this accept loop:
	```bash
	$ cargo run
	```
	In a separate terminal window, run the `hello-redis` example (the `SET`/`GET`
	command from the previous section):
	```bash
	$ cargo run --example hello-redis
	```
	The output should be:
	```text
	Error: "unimplemented"
	```
	In the server terminal, the output is:
	```text
	GOT: Array([Bulk(b"set"), Bulk(b"hello"), Bulk(b"world")])
	```

This PR follows that same pattern but for the channels section, which at the time of writing does not show the expected client/server terminal outputs at the end of the section.

doc-test is passing following these changes: