doc: correct Pstar units (CICE-Consortium#477)

* 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). Also, it already appeared in the
index with the same wrong units.

Fix that.

While at it, fix the RST syntax for `k2`.

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

* icepack: update to b1e41d9

doc/source/cice_index.rst

@@ -490,7 +490,7 @@ either Celsius or Kelvin units).
    "print_points", ":math:`\bullet` if true, print point data", "F"
    "processor_shape", ":math:`\bullet` descriptor for processor aspect ratio", ""
    "prs_sig", "replacement pressure", "N/m"
-   "Pstar", "ice strength parameter", "2.75\ :math:`\times`\ 10\ :math:`^4`\ N/m"
+   "Pstar", "ice strength parameter", "2.75\ :math:`\times`\ 10\ :math:`^4`\ N/m\ :math:`^2`"
    "puny", "a small positive number", "1\ :math:`\times`\ 10\ :math:`^{-11}`" 
    "**Q**", "", ""
    "Qa", "specific humidity at 10 m", "kg/kg"

doc/source/user_guide/ug_case_settings.rst

@@ -336,10 +336,10 @@ dynamics_nml
    "", "``1``", "transport enabled", ""
    "``Ktens``", "real", "Tensile strength factor (see :cite:`Konig10`)", "0.0"
    "``k1``", "real", "1st free parameter for landfast parameterization", "8.0"
-   "``k2``", "real", "2nd free parameter (N/m^3) for landfast parameterization", "15.0"
+   "``k2``", "real", "2nd free parameter (N/m\ :math:`^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``"