doc: update PG history as over "three decades"

Reported-by: Pierre <pbaumard@gmail.com>

Discussion: https://postgr.es/m/168724660637.399156.7642965215720120947@wrigleys.postgresql.org

Backpatch-through: 11

doc/src/sgml/history.sgml

@@ -12,7 +12,7 @@
   The object-relational database management system now known as
   <productname>PostgreSQL</productname> is derived from the
   <productname>POSTGRES</productname> package written at the
-  University of California at Berkeley.  With over two decades of
+  University of California at Berkeley.  With over three decades of
   development behind it, <productname>PostgreSQL</productname> is now
   the most advanced open-source database available anywhere.
  </para>