fix bug 597: typo in sparse documentation
fix bug 597: typo in sparse documentation

madlib/eigen_backup,rotorliu/eigen,rotorliu/eigen,madlib/eigen_backup,rotorliu/eigen,mjbshaw/Eigen,mjbshaw/Eigen,rotorliu/eigen,mjbshaw/Eigen,mjbshaw/Eigen,madlib/eigen_backup,madlib/eigen_backup