How should I connect the ground wire for my light fixture?
The instructions for the fixture are only correct for a metal box.  If a metal box was used, the box itself would (should) be grounded.  The bracket that holds the light would then be connected to the box, which would make the bracket grounded.  Finally the ground wire from the fixture would attach to the bracket, grounding the fixture.

In the case of a plastic box, the box is not grounded.  All ground connections must be made by connecting the ground wires together. So in your situation you are correct, you'll connect the ground from the supply to the ground from the fixture using a twist-on connector (or other approved connection).