correct the docstring of the grav-tensor components
correct the docstring of the grav-tensor components

victortxa/fatiando,santis19/fatiando,rafaelmds/fatiando,eusoubrasileiro/fatiando,eusoubrasileiro/fatiando,fatiando/fatiando,cmeessen/fatiando,drandykass/fatiando,eusoubrasileiro/fatiando_seismic,eusoubrasileiro/fatiando,mtb-za/fatiando,rafaelmds/fatiando,drandykass/fatiando,mtb-za/fatiando,cmeessen/fatiando,victortxa/fatiando,eusoubrasileiro/fatiando_seismic,fatiando/fatiando,santis19/fatiando