Fix typos in artist-attrs and add consequent missing batch data

[danjacka]

Thanks for this repository, it's very helpful in figuring out how to import to Datomic. Here's a fix for a couple of typos I spotted in importer.clj. These typos meant that the batch files were missing :startMonth and :startDay entries where relevant, so I ran clojure -M -m datomic.mbrainz.importer.batch subsets 100 before making this commit, to add them in.