Fix typo in docs

[janbridley]

Description

Motivation and context

Resolves #???

How has this been tested?

Checklist:

• I have reviewed the Contributor Guidelines.
• I agree with the terms of the HOOMD-blue Contributor Agreement.
• My name is on the list of contributors (sphinx-doc/credits.rst) in the pull request source branch.
• I have summarized these changes in CHANGELOG.rst following the established format.

[joaander]

Thanks for finding these. I generally prefer the way N_\mathrm{particles} looks over N_{particles}

[janbridley]

Thanks for finding these. I generally prefer the way N_\mathrm{particles} looks over N_{particles}

Updated to match this now. Here's the ruplacer command I've been using in case it's helpful ruplacer '\\?\\mathrm\{N_\{particles}-1\}' '{N_\mathrm{particles}-1}'