Minor JSON-LD fixes

- change "apod" to http://schema.org/itemListElement and make it a list
- remove erroneous @type: @id coercions (they are only for URL values)
- fix typo in "thumb" URL mapping
- change "title" and "description" (was "label") to use schema.org concepts
- mention that the file is JSON-LD in the README (if you want, you can change the extension to .jsonld to reflect that)