docs: fix progress-bar color demo

[devversion]

• Fixes the progress-bar demo in the Angular Material docs (https://material.angular.io/components/component/progress-bar)

Fixes #2813

[kara]

@devversion We are still building the docs examples from the material.angular.io repo (only halfway through the transition to this repo), so you'll need to make the change there for it actually to be reflected: https://github.com/angular/material.angular.io

That said, we will also need to keep the change in this repo for when the transition is over. I'll LGTM once it's present in both so they are kept in sync.

[devversion]

@kara Ah I thought that doing the change only in the material2 repository is fine. See docs PR

[kara]

LGTM

[kara]

@tinayuangao Can you merge angular/material.angular.io#94 before or at the same time as this one?

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