Fix ancient typo in user-defined-aggregates documentation.
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.

adam8157/gpdb,rvs/gpdb,lintzc/gpdb,50wu/gpdb,lintzc/gpdb,xinzweb/gpdb,xinzweb/gpdb,chrishajas/gpdb,janebeckman/gpdb,jmcatamney/gpdb,rvs/gpdb,kaknikhil/gpdb,xinzweb/gpdb,yuanzhao/gpdb,royc1/gpdb,adam8157/gpdb,CraigHarris/gpdb,rvs/gpdb,yuanzhao/gpdb,janebeckman/gpdb,ashwinstar/gpdb,0x0FFF/gpdb,50wu/gpdb,CraigHarris/gpdb,jmcatamney/gpdb,edespino/gpdb,xuegang/gpdb,adam8157/gpdb,50wu/gpdb,0x0FFF/gpdb,ashwinstar/gpdb,zaksoup/gpdb,ahachete/gpdb,CraigHarris/gpdb,Chibin/gpdb,50wu/gpdb,xuegang/gpdb,greenplum-db/gpdb,chrishajas/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,rvs/gpdb,lintzc/gpdb,xuegang/gpdb,ashwinstar/gpdb,zaksoup/gpdb,CraigHarris/gpdb,edespino/gpdb,xinzweb/gpdb,50wu/gpdb,cjcjameson/gpdb,xuegang/gpdb,rvs/gpdb,ahachete/gpdb,rvs/gpdb,zaksoup/gpdb,chrishajas/gpdb,royc1/gpdb,lisakowen/gpdb,lintzc/gpdb,Chibin/gpdb,edespino/gpdb,Chibin/gpdb,cjcjameson/gpdb,janebeckman/gpdb,kaknikhil/gpdb,janebeckman/gpdb,rvs/gpdb,rvs/gpdb,cjcjameson/gpdb,Quikling/gpdb,xuegang/gpdb,kaknikhil/gpdb,Quikling/gpdb,xuegang/gpdb,lintzc/gpdb,lisakowen/gpdb,edespino/gpdb,cjcjameson/gpdb,royc1/gpdb,ahachete/gpdb,ashwinstar/gpdb,edespino/gpdb,greenplum-db/gpdb,0x0FFF/gpdb,50wu/gpdb,yuanzhao/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,0x0FFF/gpdb,xinzweb/gpdb,CraigHarris/gpdb,lintzc/gpdb,edespino/gpdb,0x0FFF/gpdb,CraigHarris/gpdb,chrishajas/gpdb,jmcatamney/gpdb,lisakowen/gpdb,cjcjameson/gpdb,xuegang/gpdb,Chibin/gpdb,zaksoup/gpdb,royc1/gpdb,Chibin/gpdb,kaknikhil/gpdb,xinzweb/gpdb,greenplum-db/gpdb,cjcjameson/gpdb,50wu/gpdb,edespino/gpdb,0x0FFF/gpdb,rvs/gpdb,greenplum-db/gpdb,royc1/gpdb,chrishajas/gpdb,CraigHarris/gpdb,cjcjameson/gpdb,royc1/gpdb,adam8157/gpdb,xinzweb/gpdb,ahachete/gpdb,royc1/gpdb,ahachete/gpdb,zaksoup/gpdb,cjcjameson/gpdb,Chibin/gpdb,xinzweb/gpdb,ashwinstar/gpdb,lisakowen/gpdb,janebeckman/gpdb,lisakowen/gpdb,adam8157/gpdb,ahachete/gpdb,CraigHarris/gpdb,janebeckman/gpdb,jmcatamney/gpdb,kaknikhil/gpdb,Quikling/gpdb,janebeckman/gpdb,xuegang/gpdb,Chibin/gpdb,janebeckman/gpdb,kaknikhil/gpdb,kaknikhil/gpdb,xuegang/gpdb,yuanzhao/gpdb,edespino/gpdb,greenplum-db/gpdb,Chibin/gpdb,ashwinstar/gpdb,chrishajas/gpdb,jmcatamney/gpdb,ahachete/gpdb,yuanzhao/gpdb,Quikling/gpdb,royc1/gpdb,Chibin/gpdb,Quikling/gpdb,kaknikhil/gpdb,greenplum-db/gpdb,Quikling/gpdb,lintzc/gpdb,jmcatamney/gpdb,edespino/gpdb,50wu/gpdb,lisakowen/gpdb,zaksoup/gpdb,janebeckman/gpdb,lisakowen/gpdb,lintzc/gpdb,rvs/gpdb,greenplum-db/gpdb,ahachete/gpdb,yuanzhao/gpdb,yuanzhao/gpdb,Quikling/gpdb,0x0FFF/gpdb,jmcatamney/gpdb,cjcjameson/gpdb,adam8157/gpdb,chrishajas/gpdb,lintzc/gpdb,zaksoup/gpdb,kaknikhil/gpdb,yuanzhao/gpdb,Quikling/gpdb,yuanzhao/gpdb,Quikling/gpdb,chrishajas/gpdb,edespino/gpdb,lisakowen/gpdb,greenplum-db/gpdb,kaknikhil/gpdb,zaksoup/gpdb,Chibin/gpdb,adam8157/gpdb,0x0FFF/gpdb,Quikling/gpdb,adam8157/gpdb,janebeckman/gpdb,CraigHarris/gpdb,ashwinstar/gpdb