Update 220-contracts.md
Update 220-contracts.md

Fix the links to the portable contracts pages.
Signed-off-by: David Bosschaert <d224c28478e4b869cb6043d8fe491da8c1e1d96d@gmail.com>
rotty3000/bnd.manual,bndtools/bnd.manual,rotty3000/bnd.manual,bndtools/bnd.manual,bndtools/bnd.manual,rotty3000/bnd.manual