doc: Fix date in EPUB manifest

If there is no <date> element, the publication date for the EPUB
manifest is taken from the copyright year.  But something like
"1996-2013" is not a legal date specification.  So the EPUB output
currently fails epubcheck.

Put in a separate <date> element with the current year.  Put it in
legal.sgml, because copyright.pl already instructs to update that
manually, so it hopefully won't be missed.

doc/src/sgml/legal.sgml

 <!-- doc/src/sgml/legal.sgml -->
 
+<date>2013</date>
+
 <copyright>
  <year>1996-2013</year>
  <holder>The PostgreSQL Global Development Group</holder>