Makefile: fix darwin folder path in git-cola.app recipe

cp command was referencing old darwin folder.
darwing folder is now located inside contrib.

Signed-off-by: Maicon D. Filippsen <maiconfilippsen@gmail.com>
Merge pull request #230 from filippsen/makefile-fix

Makefile: fix darwin folder path in git-cola.app recipe

Signed-off-by: David Aguilar <davvid@gmail.com>