[RM] A correction in the derivative of fluid compressiblity with respect to pressure

ProcessLib/RichardsMechanics/RichardsMechanicsFEM-impl.h

@@ -964,7 +964,8 @@ void RichardsMechanicsLocalAssembler<ShapeFunctionDisplacement,
         double const specific_storage_a_S = phi - p_cap_ip * S_L * a0;
 
         double const dspecific_storage_a_p_dp_cap =
-            dS_L_dp_cap * (phi * beta_LR + 2 * S_L * a0);
+            -dS_L_dp_cap * (phi * beta_LR + 2 * S_L * a0) -
+            S_L * phi * beta_LR * beta_LR;
         double const dspecific_storage_a_S_dp_cap =
             -a0 * (S_L + p_cap_ip * dS_L_dp_cap);