Adding Plasma File in TOC

[isaacgsmith]

Fixing a mistake in PR #1371.

Description

After renaming a file in PR #1371, docs/index.rst was not updated, and therefore docs/physics/plasma.rst does not show up in the main TOC. This change fixes it.

Motivation and Context

I opened an issue for this-- #1377 .

How Has This Been Tested?

I did test this, and plasma does appear in the sidebar.

Screenshots (if appropriate):

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have assigned/requested two reviewers for this pull request.

[codecov]

Codecov Report

Merging #1378 (5236056) into master (1c706d5) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1378   +/-   ##
=======================================
  Coverage   72.39%   72.39%           
=======================================
  Files          66       66           
  Lines        5097     5097           
=======================================
  Hits         3690     3690           
  Misses       1407     1407           

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