fix docs

Author: ranocha

docs/src/dingemans.md

@@ -289,7 +289,7 @@ hysgn = HyperbolicSerreGreenNaghdiEquations1D(bathymetry_type = bathymetry_mild_
                                               # and the SGN equation.
 
 hysm = HyperbolicSainteMarieEquations1D(bathymetry_type = bathymetry_mild_slope,
-                                        gravity = 9.81, eta0 = 0.8)
+                                        gravity = 9.81, eta0 = 0.8, h0 = 0.8)
 
 initial_condition = initial_condition_dingemans
 boundary_conditions = boundary_condition_periodic