Fix: Combobox Disabled propType

Fixes #2988

There is a propTypes.txt file that contains a reference to PropTypes.boolean, but I'm unclear how that is generated.

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[x] npm run lint:fix has been run and linting passes.
[x] Mocha, Jest (Storyshots), and components/component-docs.json CI tests pass (npm test).
[x] Tests have been added for new props to prevent regressions in the future. See readme.
[x] Review the appropriate Storybook stories. Open http://localhost:9001/.
[x] Review tests are passing in the browser. Open http://localhost:8001/.
[x] Review markup conforms to SLDS by looking at DOM snapshot strings.

[ ] CircleCI tests pass. This includes linting, Mocha, Jest, Storyshots, and components/component-docs.json tests.
[ ] Tests have been added for new props to prevent regressions in the future. See readme.
[ ] Review the appropriate Storybook stories. Open http://localhost:9001/.
[ ] The Accessibility panel of each Storybook story has 0 violations (aXe). Open http://localhost:9001/.
[ ] Review tests are passing in the browser. Open http://localhost:8001/.
[ ] Review markup conforms to SLDS by looking at DOM snapshot strings.
Required only if there are markup / UX changes
[ ] Add year-first date and commit SHA to last-slds-markup-review in package.json and push.
[ ] Request a review of the deployed Heroku app by the Salesforce UX Accessibility Team.
[ ] Add year-first review date, and commit SHA, last-accessibility-review, to package.json and push.
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salesforce / design-system-react