[doxygen] fix doxygen in fluidstates

dumux/material/fluidstates/2p2c.hh

dumux/material/fluidstates/CMakeLists.txt

dumux/material/fluidstates/compositional.hh

dumux/material/fluidstates/immiscible.hh

dumux/material/fluidstates/isothermalimmiscible.hh

dumux/material/fluidstates/nonequilibrium.hh

dumux/material/fluidstates/nonequilibriumenergy.hh

dumux/material/fluidstates/nonequilibriummass.hh

dumux/material/fluidstates/pressureoverlay.hh

dumux/material/fluidstates/pseudo1p2c.hh

dumux/material/fluidstates/saturationoverlay.hh

dumux/material/fluidstates/temperatureoverlay.hh