add end-quote in index

[eclare108213]

• Short (1 sentence) summary of your PR:
  Missing end-quote was preventing documentation index from rendering correctly
• Developer(s):
  @eclare108213
• Suggest PR reviewers from list in the column to the right.
• Please copy the PR test results link or provide a summary of testing completed below.
  See readthedocs below or direct link: https://cice-consortium-icepack--547.org.readthedocs.build/en/547/appendices/icepack_index.html
• How much do the PR code changes differ from the unmodified code?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on CICE or any other models?
  • Yes
  • No
• Does this PR add any new test cases?
  • Yes
  • No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please document the changes in detail, including why the changes are made. This will become part of the PR commit log.
  A one-character change to fix the index of variables in readthedocs.