Update Sprockets configuration examples

As of middleman-sprockets v3.3.2, wrapping `sprockets.append_path` or
`sprockets.import_asset` in a `ready` block is no longer necessary.

See: https://github.com/middleman/middleman-sprockets/blob/151a83f2b6f26758fb392cb8d1d315765ab7ab4f/CHANGELOG.md