helpers.rb: Fix typo in :contributor keyword
helpers.rb: Fix typo in :contributor keyword

Typo in query_terms stops creator|contributor|author field name look up
crosscite/doi-metadata-search,crosscite/doi-metadata-search,crosscite/doi-metadata-search,sci-gaia/doi-metadata-search,sci-gaia/doi-metadata-search,sci-gaia/doi-metadata-search