change url so that it isn't separate from the marketing site

[LeighKP]

Request:
Please change the url for Pressbooks Directory from pressbooks.directory to pressbooks.com/directory

Why:
To improve SEO and better integrate Pressbooks Directory into our marketing site's web presence.

Other notes:
This was a suggestion from one of @hughmcguire 's advisors (whose name escapes me) and I agree. Pressbooks, because it has been around so long, has good SEO cred and it would be great if the directory could benefit from that.