Update Alfa Slab One to the current version

This gets us multiple things that are fixed in the current version:

1. Fixes the `s` (which is very prominent on the front page) to include
   serifs and better visually match the other glyphs.
2. Improves the inter-letter spacing.
3. Fixes #858 by including the `ğ` glyph used in Turkish, for example on
   https://www.rust-lang.org/tr/learn

Author: LawnGnome