fixes geopotential gradient term in governing eqs.

docs/equations/governing.rst

@@ -6,7 +6,7 @@ Governing equations
 The Boussinesq hydrostatic equations of motion in height coordinates are
 
 .. math::
-  D_t \boldsymbol{u} + f \widehat{\boldsymbol{k}} \wedge \boldsymbol{u} + \boldsymbol{\nabla}_z \Phi + \frac{1}{\rho_o} \boldsymbol{\nabla}_z p &= \boldsymbol{\nabla} \cdotp \boldsymbol{\underline{\tau}} , \\
+  D_t \boldsymbol{u} + f \widehat{\boldsymbol{k}} \wedge \boldsymbol{u} + \frac{\rho}{\rho_o} \boldsymbol{\nabla}_z \Phi + \frac{1}{\rho_o} \boldsymbol{\nabla}_z p &= \boldsymbol{\nabla} \cdotp \boldsymbol{\underline{\tau}} , \\
   \rho \partial_z \Phi + \partial_z p &= 0 , \\
   \boldsymbol{\nabla}_z \cdotp \boldsymbol{u} + \partial_z w &= 0 , \\
   D_t \theta &= \boldsymbol{\nabla} \cdotp \boldsymbol{Q}_\theta , \\