Icon-step images are in a folder

The icon-step images were moved into a folder[1]. Update the Sass to look
for them in that folder.

[1]: https://github.com/alphagov/govuk_frontend_toolkit/commit/e523d51536482f3f300b798883c348e9d41bf2da
Merge pull request #1774 from alphagov/icon-steps

Icon-step images are in a folder