Enable MDBOOK_BOOK to overwrite book.toml

[toyboot4e]

Config::update_from_env says:

/// > This means, if you so desired, you could override all book metadata
/// > when building the book with something like
/// >
/// > ```text
/// > $ export MDBOOK_BOOK="{'title': 'My Awesome Book', authors: ['Michael-F-Bryan']}"
/// > $ mdbook build
/// > ```

This is wrong in two ways:

1. We can't overwrite book.toml with MDBOOK_BOOK
2. The example value of MDBOOK_BOOK is a invalid JSON string (missing/unproper quotes)

So I fixed them so that the example works. But I didn't add any automated test.

[toyboot4e]

Thank you very much for your review!

[ehuss]

Thanks!