Fix #638955

Connect the `Help' button in the `Close Book' dialog box (Tools > Close
Book) to newly-written help section on book closing.

Signed-off-by: Yawar Amin <yawar.amin@gmail.com>

git-svn-id: svn+ssh://svn.gnucash.org/repo/gnucash/branches/2.4@22702 57a11ea4-9604-0410-9ed3-97b8803252fd