Fix native range thumb styles on Webkit (Chrome/Safari)

Fix native range thumb styles on Webkit (Chrome/Safari)

The range slider's thumb was not styling correctly on Chrome or Safari.
This was simply because the pseudo-element `::-webkit-slider-thumb`
had been incorrectly written as `::-wekit-slider-handle` in `scss/forms/_range.scss`.

This revision fixes it to correctly use `::-webkit-slider-thumb`.

Test Plan:
- Run `npm start`
 - View `slider.html` in the docs in Chrome and check the range
   slider's thumb is now styled correctly.
 - View the docs in Firefox and Edge and ensure they are still correct.
- Run `npm run test:sass` and ensure all the tests still pass