1.2. Add Reference to dead_code

[LingkKang]

I think add a little explanation of this #[allow(dead_code)] could be better. One line is enough.

rust-by-example/src/hello/print.md

Lines 62 to 68 in 8888f94

	#[allow(dead_code)]
	struct Structure(i32);
	// This will not compile because `Structure` does not implement
	// fmt::Display.
	//println!("This struct `{}` won't print...", Structure(3));
	// TODO ^ Try uncommenting this line

Firstly, I'm a little bit confused by it, and then I googled. The first link redirected me to dead_code - Rust By Example.

Since it would be covered later in RBE, why don't mention it and give a reference link? Just like giving out a link of macro in Hello World - Rust By Example.

This could make life much easier for beginners.