Fix "Boundary temperature model" documentation

[melaniegerault]

A minor thing, but I have noticed that the "box with lithosphere boundary indicators" option that is indicated on the website (see below) and in the manual is not included in the source code anymore. It seems like it could be the same as two_merged_boxes.cc

[naliboff]

@melaniegerault - Thanks for brining this up. The box with lithosphere boundary indicators initial composition, initial temperature and geometry models are in fact the same as the two_merged_boxes models (e.g. same files, the name box with ... is only assigned when the model is registered).

The original naming scheme may have been kept within the two_merged_boxes models to avoid confusion regarding loss of a feature?

[melaniegerault]

I see, thanks. That was just in case it wasn't on purpose :)