update pagc install instructions

git-svn-id: http://svn.osgeo.org/postgis/trunk@11699 b70326c6-7e19-0410-871a-916f4a2858ee
typo

git-svn-id: http://svn.osgeo.org/postgis/trunk@11700 b70326c6-7e19-0410-871a-916f4a2858ee
more clarification

git-svn-id: http://svn.osgeo.org/postgis/trunk@11701 b70326c6-7e19-0410-871a-916f4a2858ee