Add correct license for asciidoctor

Asciidoctor has a MIT license [1], not an Apache 2.0 license.

[1] https://github.com/asciidoctor/asciidoctor/blob/master/LICENSE.adoc

Change-Id: Id7c566685c9ac070fefe5f785c78e29be04c7a43
Signed-off-by: Edwin Kempin <ekempin@google.com>