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.
Replacing all references to the 2nd person with references to the 3rd
person (excluding `authors = [ "Your name <you@example.com>" ]` and
`file:///home/yourname/projects/hello_world` in `hello-cargo.md`)
