fix bad image asset paths

Due to “refactor grunt build” commit:
https://github.com/jennschiffer/make8bitart/commit/52c21f5a72e0aed7e5dab
6b2437fc5d53599b958

Since the asset paths in the Gruntfile were updated, building locally via `grunt` results in a couple images and one font not loading. Live site is fine since file structure is probably built prior to the above-mentioned commit.