GeneticEpi_Matrix.rst refers to undefined matrices

This portion of the user guide refers to the matrices 'expMeanMZ' and 'expMeanDZ' in several places. However these matrices don't exist. See http://openmx.psyc.virginia.edu/docs/OpenMx/latest/GeneticEpi_Matrix.html. Make sure to svn update before making changes, as I've made some other corrections to the file.