Corrected variable name swndf, swndr to swidf, swidr in Icepack doc

[daveh150]

… in CICE code

For detailed information about submitting Pull Requests (PRs) to the CICE-Consortium,
please refer to: https://github.com/CICE-Consortium/About-Us/wiki/Resource-Index#information-for-developers

PR checklist

• [ x ] Short (1 sentence) summary of your PR:
  Corrected variable names 'swndf, swndr' to 'swidf, swidr' in documentation. swidf, swidr are used in Icepack/CICE.
• [ x ] Developer(s):
  David Hebert
• [ x ] 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.
  ENTER INFORMATION HERE
• 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
  • [ x ] No
• Does this PR add any new test cases?
  • Yes
  • [ x ] 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/.)
  • [ x ] Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• [ X ] Please provide any additional information or relevant details below:
• This is only in Icepack documentation. CICE documentation is correct.
• As this is a simple documentation change, test suite not run.