eina_matrix: add missing "*" for doxygen

eina_matrix: add missing "*" for doxygen

========================================================================
    eina_matrix: add missing "*" for doxygen

    Summary:
    "/**" requires for doxygen, but one "*" is omitted for the reference
    of eina_matrix3_multiply().

    Reviewers: Hermet

    Reviewed By: Hermet

    Subscribers: cedric, jpeg

    Differential Revision: https://phab.enlightenment.org/D4266
========================================================================

Signed-Off-By: Jee-Yong Um <jc9.um@samsung.com>
Change-Id: I3fea00228414a0aa976277ce78f1dce2eb191f23