doc: fix 'ice strength' description

[phil-blain]

PR checklist

• Short (1 sentence) summary of your PR:
  Correct description of 'ice strength' in the index
• Developer(s):
  P Blain
• 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.
  Doc fix; no tests.
• 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 Icepack 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/. A test build of the technical docs will be performed as part of the PR testing.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No

[phil-blain]

See also CICE-Consortium/Icepack#336

[apcraig]

For some reason, the readthedocs status is not getting a check mark. I have confirmed the documentation builds fine and that the doc change is there. I will merge the PR now. I don't see any reason to force push to reset the status checks in this case.