kiji-schema: SCHEMA-649. Tiny bug in kiji-schema-extras Avro value parser

 - Fix typo in unit-test

issue: https://jira.kiji.org/browse/SCHEMA-649
review: https://review.kiji.org/r/1619
commit: https://github.com/kijiproject/kiji-schema/commit/ed68c17d18d17e742ebd604f55dbc83e2b30db83