Fix symbol for water content in IC user docs

Add subscript "w" for clarification

doc/manual/initial.rst

@@ -103,8 +103,8 @@ Initial condition tranformations for the Richards solver.
     * ``quantity = waterContent``
 
     The input data is interpreted as water content,
-    :math:`f = \theta \, [\text{-}]`, and transformed into matric head via the
-    :doc:`parameterization <parameter-file>` of the medium.
+    :math:`f = \theta_w \, [\text{-}]`, and transformed into matric head via
+    the :doc:`parameterization <parameter-file>` of the medium.
 
     Values greater than the porosity :math:`\phi` and less than the residual
     water content :math:`\theta_r` are automatically clamped to fit the allowed