Update index.d.ts to match library documentation

[jdedwards3]

see this related issue that is now closed:
leongersen/noUiSlider#813

When calling get type should be strings

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Select one of these and delete the others:

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[typescript-bot]

@jimwards17 Thank you for submitting this PR!

🔔 @bleuarg @lagaffe - please review this PR in the next few days. Be sure to explicitly select Approve or Request Changes in the GitHub UI so I know what's going on.

If no reviewer appears after a week, a DefinitelyTyped maintainer will review the PR instead.

[typescript-bot]

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