Added description of `@`
Added description of `@`

In the book, this is explicitly stated. In rustbyexample, confusion may
occur without thorough reading of the code

chriskrycho/rust-by-example,jinyuanxie/rust-by-example-cn,mdinger/rust-by-example,rust-lang-cn/rust-by-example-cn,jinyuanxie/rust-by-example-cn,chriskrycho/rust-by-example,realcr/rust-by-example,aakloxu/rust-by-example-cn,nodakai/rust-by-example,aakloxu/rust-by-example-cn,nodakai/rust-by-example,mdinger/rust-by-example,rust-lang-cn/rust-by-example-cn,realcr/rust-by-example