Update fix_rigid.rst

.. index:: fix rigid

.. index:: fix rigid/omp

.. index:: fix rigid/nve

.. index:: fix rigid/nve/omp

.. index:: fix rigid/nvt

.. index:: fix rigid/nvt/omp

.. index:: fix rigid/npt

.. index:: fix rigid/npt/omp

.. index:: fix rigid/nph

.. index:: fix rigid/nph/omp

.. index:: fix rigid/small

.. index:: fix rigid/small/omp

.. index:: fix rigid/nve/small

.. index:: fix rigid/nvt/small

.. index:: fix rigid/npt/small

.. index:: fix rigid/nph/small

fix rigid command

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fix rigid/omp command

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Accelerator Styles: *rigid/omp*

fix rigid/nve command

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fix rigid/nve/omp command

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Accelerator Styles: *rigid/nve/omp*

fix rigid/nvt command

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

fix rigid/nvt/omp command

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Accelerator Styles: *rigid/nvt/omp*

fix rigid/npt command

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

fix rigid/npt/omp command

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

Accelerator Styles: *rigid/npt/omp*

fix rigid/nph command

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

fix rigid/nph/omp command

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Accelerator Styles: *rigid/nph/omp*

fix rigid/small command

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fix rigid/small/omp command

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Accelerator Styles: *rigid/small/omp*

fix rigid/nve/small command

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