fix doc mistake

Author: damiano-flex

tidy3d/components/tcad/boundary/heat.py

@@ -133,7 +133,7 @@ class ConvectionBC(HeatChargeBC):
     >>>
     >>> # Next, create the model, which requires the fluid and a characteristic length.
     >>> natural_conv_model = td.VerticalNaturalConvectionCoeffModel.from_si_units(
-    ...     medium=air, characteristic_length=1e-5
+    ...     medium=air, plate_length=1e-5
     ... )
     >>>
     >>> # Finally, create the boundary condition using this model.
@@ -143,7 +143,7 @@ class ConvectionBC(HeatChargeBC):
 
     >>> # If the fluid medium is not provided to the coefficient model, it is automatically retrieved from
     >>> # the interface.
-    >>> natural_conv_model_nom = td.VerticalNaturalConvectionCoeffModel.from_si_units(characteristic_length=1e-5)
+    >>> natural_conv_model_nom = td.VerticalNaturalConvectionCoeffModel.from_si_units(plate_length=1e-5)
     >>> bc_natural_nom = td.ConvectionBC(
     ...     ambient_temperature=300, transfer_coeff=natural_conv_model
     ... )