Add margin-left to sort button in sketches page

[jeanetteandrews]

Fixes #3432

Changes:
Added margin-left: #{math.div(5, $base-font-size)}rem; to .sketch-list__sort-button svg in _sketch-list.scss for better spacing of the sort button on the 'Sketches' page.

This ensures consistency with the existing code in _nav.scss, where the same margin is applied to .nav__item-header-triangle on the 'Editor' page.

I have verified that this pull request:

• has no linting errors (npm run lint)
• has no test errors (npm run test)
• is from a uniquely-named feature branch and is up to date with the develop branch.
• is descriptively named and links to an issue number, i.e. Fixes #123

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[raclim]

Thanks for much for fixing this @jeanetteandrews, this looks great to me!