Update fix_nh.rst

.. index:: fix nvt

.. index:: fix nvt/intel

.. index:: fix nvt/kk

.. index:: fix nvt/omp

.. index:: fix npt

.. index:: fix npt/intel

.. index:: fix npt/kk

.. index:: fix npt/omp

.. index:: fix nph

.. index:: fix nph/kk

.. index:: fix nph/omp

fix nvt command

===============

fix nvt/intel command

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fix nvt/kk command

==================

fix nvt/omp command

===================

Accelerator Styles: *nvt/intel*, *nvt/kk*, *nvt/omp*

fix npt command

===============

fix npt/intel command

=====================

fix npt/kk command

==================

fix npt/omp command

===================

Accelerator Styles: *npt/intel*, *npt/kk*, *npt/omp*

fix nph command

===============

fix nph/kk command

==================

fix nph/omp command

===================

Accelerator Styles: *nph/kk*, *nph/omp*

Syntax

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