Repository: rust-lang/mdBook (language: rust)

Issue:
Docs: Please fix MDBOOK_BOOK example
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. 


Discussion / hints:

Fix patch:
diff --git a/guide/src/format/configuration/environment-variables.md b/guide/src/format/configuration/environment-variables.md
index 3631769bea..8799c54874 100644
--- a/guide/src/format/configuration/environment-variables.md
+++ b/guide/src/format/configuration/environment-variables.md
@@ -29,7 +29,7 @@ book's title without needing to touch your `book.toml`.
 > building the book with something like
 >
 > ```shell
-> $ export MDBOOK_BOOK="{'title': 'My Awesome Book', authors: ['Michael-F-Bryan']}"
+> $ export MDBOOK_BOOK='{"title": "My Awesome Book", "authors": ["Michael-F-Bryan"]}'
 > $ mdbook build
 > ```
 


Test patch: