Fix headings for book

Some Markdown files start with

    ---
    layout: lesson
    root: .
    title: TITLE
    ---
    ### SUBTITLE

When creating `book.md` it will have

    ## TITLE
    ### SUBTITLE

that is translate into HTML by Jekyll as

    <h2>TITLE</h2>
    <p>### SUBTITLE</p>

Change `make-book.py` to generate `book.md` having

    ## TITLE

    ### SUBTITLE

will fix this issue without side effect.

Thanks to @jduckles for reporting this issue.
Merge pull request #545 from r-gaia-cs/fix-book-h2

Fix headings for book