icepack_therm_shared: correct 'Tair' units in declaration comment (#399)

In subroutine icepack_therm_shared::icepack_init_trcr, 'Tair' is
documented as having units 'C' (degrees Celsius), but this is incorrect,
this variable is in Kelvins. This can be seen a few lines below where we
compute Tsfc:

    Tsfc = min(Tsmelt, Tair - Tffresh) ! deg C

where 'Tffresh' is the freezing point of fresh ice in Kelvins, 273.15,
as defined in icepack_parameters.

Fix the units in the declaration comment.

Reported-by: Frederic Dupont <[email protected]>

columnphysics/icepack_therm_shared.F90

@@ -295,7 +295,7 @@ subroutine icepack_init_trcr(Tair,     Tf,       &
          nslyr       ! number of snow layers
 
       real (kind=dbl_kind), intent(in) :: &
-         Tair, &     ! air temperature (C)
+         Tair, &     ! air temperature (K)
          Tf          ! freezing temperature (C)
 
       real (kind=dbl_kind), dimension(:), intent(in) :: &