Fixed #20670 -- Fixed project layout in reusable apps tutorial.

Backport of 70d3adf9f2ebfdfc9e64376b71b596ebb12d77f8 from master.
Thanks to nicklasp for the report.
[1.5.x] Fixed missing slash in reusable apps tutorial.

Backport of 5caced89e0ac2f942b68bc5f163d156a42880f16 from master.