Correct link for chapter 4 of User Stories Applied

It's not helpful to link to a free PDF of chapter 2 of User Stories
Applied, which doesn't contain the information needed. Better to link to
a page summarising the book and telling them where they can get it.
Merge pull request #149 from alphagov/correct-reference-link-user-stories-applied-chapter-four

Correction of link on Writing User Stories page