Fix problem handling `examples` and `interaction`.

> @soegaard reported this to me yesterday on IRC. He noticed it [here](https://github.com/soegaard/bind/blob/master/README.md).

Unlike plain `racketblock`, `examples` and `interaction` are
"nested". As a result we emitted bad Markdown like:

    ```racket
    Examples:
    ```racket
    some-code
    ```
    ```

Markdown code blocks can't nest, so this needs to be:

    ```racket
    Examples:
    some-code
    ```

Also: Updated the unit test with examples of `examples` and
`interaction`.