Update main.rs to show the full example

[Hunter-Plus]

It's better if readers can also see "let (tx, mut rx) = trpl::channel();" in this example to align with others in this chapter. By doing this, beginners will have less unnecessary confusions when trying to run the demo.

[rickrain]

I recall this when I read it the first time and think it would be helpful. After all, it is just 1 line of code and therefore not saving much screen real estate.

Readers do have the option (at any time) to review the full source listing though.

Not sure what the right balance is but I specifically remembered this when I went through it the first time.

[Hunter-Plus]

I recall this when I read it the first time and think it would be helpful. After all, it is just 1 line of code and therefore not saving much screen real estate.

Readers do have the option (at any time) to review the full source listing though.

Not sure what the right balance is but I specifically remembered this when I went through it the first time.

Thank you for your attention on this! I think the point is to make the hidden lines to align with other examples in this chapter to avoid any confusion for beginners. Also, we can hide this line of other examples to save more screen except for the first one. Please feel free to consider my change or close this PR and use other alternatives~

Rust book is definitely the best tutorial I've ever read, appreciate your efforts on it! Have a wonderful weekend ahead!