Fixed wrong reference in handbook: The problem description (treatment of...

Fixed wrong reference in handbook: The problem description (treatment of Boundary Condition) in the decoupled tutorial references common parts of the coupled tutorial. To indicate where the description in the coupled part becomes model-specific again, the phrase "box models" was added.

git-svn-id: svn://svn.iws.uni-stuttgart.de/DUMUX/dumux/trunk@10106 2fb0f335-1f38-0410-981e-8018bf24f1b0