Fix links to glossary at book.html

@gvwilson Could you take a look on it?

Originally the glossary is available at
http://software-carpentry.org/v5/gloss.html but for
software-carpentry.org/v5/book.html we is already available at
this document and we NEED make the links point to it (and not
http://software-carpentry.org/v5/gloss.html) to avoid problems
when creating the EPUB and PDF version of our lessons.