At this point of the book, reader have likely use `cargo new --bin`,
likely 2 times, once if they are lazy. This remind them of the `cargo`
syntax.
I was myself unsure whether it was `cargo create`, `cargo new`, and
whether it would initialize in current working directory or needed a
target.
The book was located under 'src/doc/trpl' because originally, it was
going to be hosted under that URL. Late in the game, before 1.0, we
decided that /book was a better one, so we changed the output, but
not the input. This causes confusion for no good reason. So we'll change
the source directory to look like the output directory, like for every
other thing in src/doc.
