Update books.rst (for validation)

Per discussion with Roland and Kathleen, suggest we point users interested in the Akka in Action book to the Typesafe.com 'Akka in Action' E-Book page, so that they can obtain free sample chapters.
(cherry picked from commit b17142f368fb07b34c3bed2727eb62a77790866f)

Conflicts:
	akka-docs/rst/additional/books.rst