Add cargo commands to the tutorial

[Keksoj]

As discussed in #503 , the tutorial becomes confusing around the Channel section. I have idenitfied it as a lack of instructions on structuring the code.

Following the pattern of instructions of the Spawning page (create an examples/ directory, rename main.rs into it), I have edited the tutorial so it suggests to rename / create binary files in a bin directory:

• src/bin/server.rs
• src/bin/client.rs
• src/bin/echo-server-copy.rs
• src/bin/echo-server.rs

With all the additional run instructions: cargo run --bin … and some verbosity.

I believe this will help learners like me who like to type code and tend to be reassured when told where and how.

[Darksonn]

Thanks!

[Darksonn]

Suggested change

	(We won't really try out the server, but feel free to build your own little client using
	[tokio::net::TcpStream example](https://docs.rs/tokio/1.4.0/tokio/net/struct.TcpStream.html#examples)
	with the proper socket address.)
	You will be able to try the server using a standard command-line tool such as `telnet`, or by writing
	a simple client like the one found in the documentation for [`tokio::net::TcpStream`][tcp_example].
	[tcp_example]: https://docs.rs/tokio/1/tokio/net/struct.TcpStream.html#examples

[Darksonn]

You will need to change this to a fenced code block for our CI setup to not consider this as Rust code that it can run.

```text
mkdir src/bin
mv src/main.rs src/bin/server.rs
```

[Keksoj]

I put backticks but the CI still fails. Can't find the bug :/ oh. Three backticks + text. Got it. On it.

[Keksoj]

Looks ready to me.

[Darksonn]

Thanks.