Fix bug in sparse documentation.
(transplanted from fdd926b87a06e08fa850ef403e7d149ddbd7216c)

--HG--
branch : 3.1
extra : transplant_source : %FD%D9%26%B8z%06%E0%8F%A8P%EF%40%3E%7D%14%9D%DB%D7%21l