support modified date in EPUB3 meta element

In http://idpf.org/epub/30/spec/epub30-publications.html#sec-meta-elem,

    "the metadata element must contain exactly one meta element
     defining a [DCTERMS] modified property for the Publication."

I'm not sure whether 'event' value should be "modification" or "modified" (I use "modification" which is used in EPUB2).