Update fix_nh.rst

.. math::

   W = (N + 1) k T_{target} P_{damp}^2

   W = (N + 1) k T_{\rm target} P_{\rm damp}^2

where :math:`N` is the number of atoms, :math:`k` is the Boltzmann constant, 

and :math:`T_{target}` is the target temperature of the barostat :ref:`(Martyna) <nh-Martyna>`.

If a thermostat is defined, :math:`T_{target}` is the target temperature

of the thermostat. If a thermostat is not defined, :math:`T_{target}`

and :math:`T_{\rm target}` is the target temperature of the barostat :ref:`(Martyna) <nh-Martyna>`. 

If a thermostat is defined, :math:`T_{\rm target}` is the target temperature 

of the thermostat. If a thermostat is not defined, :math:`T_{\rm target}` 

is set to the current temperature of the system when the barostat is initialized. 

If this temperature is too low the simulation will quit with an error. 

Note: in previous versions of LAMMPS, :math:`T_{target}` would default to

Note: in previous versions of LAMMPS, :math:`T_{\rm target}` would default to 

a value of 1.0 for *lj* units and 300.0 otherwise if the system had a temperature 

of exactly zero. 

If a thermostat is not specified by this fix, :math:`T_{target}` can be

If a thermostat is not specified by this fix, :math:`T_{\rm target}` can be 

manually specified using the *Ptemp* parameter. This may be useful if the 

barostat is initialized when the current temperature does not reflect the 

steady state temperature of the system. This keyword may also be useful in