docs/cask: replace single-quoted with double-quoted

[bevanjkay]

• Have you followed the guidelines in our Contributing document?
• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your changes? Here's an example.
• Have you successfully run brew style with your changes locally?
• Have you successfully run brew typecheck with your changes locally?
• Have you successfully run brew tests with your changes locally?

---

Replaces references to "single-quoted" strings with "double-quoted" strings to match our style guide.

[carlocab]

Thanks @bevanjkay!