Docs: Please fix MDBOOK_BOOK example

[lgersman]

Hi guys,

in mdBook/guide/src/format/configuration/environment-variables.md is a code example showing how to use JSON to override title and stuff when running mdbook build.

$ export MDBOOK_BOOK="{'title': 'My Awesome Book', authors: ['Michael-F-Bryan']}"
$ mdbook build

The export line is wrong, it has to be

export MDBOOK_BOOK='{"title": "My Awesome Book", "authors": ["Michael-F-Bryan"]}'

otherwise it's not JSON :-)

I invested an hour to get the this nice feature working but the docs where wrong ...
Now the example works like a charm.

PS: By the way - I love mdbook, great software ! Simple, fast, functional.