added reference in retstable.Rd; minor corrections

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/nacopula@1076 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
minor correction

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/nacopula@1077 edb9625f-4e0d-4859-8d74-9fd3b1da38cb