Fix ancient typo in user-defined-aggregates documentation.

The description of the initcond value for the built-in avg(float8)
aggregate has been wrong since it was written.  Noted by Disc Magnet.

doc/src/sgml/xaggr.sgml

@@ -106,7 +106,7 @@ CREATE AGGREGATE avg (float8)
     sfunc = float8_accum,
     stype = float8[],
     finalfunc = float8_avg,
-    initcond = '{0,0}'
+    initcond = '{0,0,0}'
 );
 </programlisting>
   </para>