Fix open-sans font filenames

The Upstream repo changed the casing of the SemiBold from Semibold. Tho
it was not changed in all locations[^1]. Going back to the comit prior
to the change fixes the 404 on the font files.

[^1]: Respective PRs have been open since 2018 and 2019:
FontFaceKit/open-sans#36,
FontFaceKit/open-sans#37