Fix typo in corrective type mapping in transform.clj

[alex-ball]

This typo probably accounts for the behaviour noted by by @larsgw in CrossRef/rest-api-doc#187 (comment).

[larsgw]

I don't know, I'm also still getting book-chapter for 10.1007/978-3-319-11964-9_7 which should be covered already looking at these changes. (Also, the type should be conference paper, but that might be due to the data delivered to Crossref rather than the API.)