Use more descriptive reference style (#138)

Change xrefstyle to short

Reference:
https://github.com/asciidoctor/asciidoctor.org/blob/master/docs/_includes/xrefstyle.adoc

After some discussion and looking at several technical books, this seems
to be the popular style, and picking this one for now.

book.asciidoc

@@ -15,6 +15,7 @@
 :ditaa-shadows: false
 :ditaa-transparent: true
 :attribute-missing: warn
+:xrefstyle: short
 
 include::chapters/preface.asciidoc[]