`register_descriptor` docs have incorrect type for `datavalues`

The example in the docs for `register_descriptor` suggests that `datavalues` is an array of dicts, when the actual type is a dict. This confused me for a while when working on descriptor registration for HWI.

Locate the documentation example for register_descriptor and review how datavalues is described. Correct the example so datavalues is shown as a dict rather than an array of dicts, then verify the documented type matches the API used for descriptor registration in HWI.

Written by the indexing model from the issue text.