change 'maths'->'math' in doxygen doc
change 'maths'->'math' in doxygen doc

hsorby/libcellml,libcellml/buildbot-testing,hsorby/libcellml,MichaelClerx/libcellml,MichaelClerx/libcellml,nickerso/libcellml,cellml/libcellml,cellml/libcellml,hsorby/libcellml,cellml/libcellml,nickerso/libcellml,MichaelClerx/libcellml,libcellml/buildbot-testing,nickerso/libcellml,nickerso/libcellml,hsorby/libcellml,cellml/libcellml