doc: correct 'Pstar' units

'Pstar' was added to the namelist in c9cfebd (add Pstar and Cstar to
namelist (CICE-Consortium#475), 2020-06-26), but the wrong units were added to the
documentation (N/m instead of N/m^2).

Fix that.

Reported-by: Jean-François Lemieux <jean-francois.lemieux@canada.ca>

doc/source/user_guide/ug_case_settings.rst

@@ -339,7 +339,7 @@ dynamics_nml
    "``k2``", "real", "2nd free parameter (N/m^3) for landfast parameterization", "15.0"
    "``mu_rdg``", "real", "e-folding scale of ridged ice for ``krdg_partic`` = 1 in m^0.5", "3.0"
    "``ndte``", "integer", "number of EVP subcycles", "120"
-   "``Pstar``", "real", "constant in Hibler strength formula (N/m)", "2.75e4"
+   "``Pstar``", "real", "constant in Hibler strength formula (N/m\ :math:`^2`)", "2.75e4"
    "``revised_evp``", "logical", "use revised EVP formulation", "``.false.``"
    "``threshold_hw``", "real", "Max water depth for grounding (see :cite:`Amundrud04`)", "30."
    "``yield_curve``", "``ellipse``", "elliptical yield curve", "``ellipse``"