Update aou2marcxml.xsl - publication date for "proceedings chapters" in 260-c instead of 773-y
requested to merge Jean-Blaise.Claivaz/aou-backend:Jean-Blaise.Claivaz-master-patch-33875 into master
Publication date for "proceedings chapters" in 260-c instead of 773-y. Mistake coming from the fact that proc-chapter were sub-type of books previously, but are sub-type of conference now.