Measurements Block: minor bug in documentation

The `Measurement` python class has field "value", and not "measurement_value"

- however, on the website it says "measurement_value": https://phenopacket-schema.readthedocs.io/en/latest/measurement.html#rstmeasurement

Open the linked Measurement documentation page and compare its field name with the Python Measurement class. Update the documentation from measurement_value to value, then verify that the rendered page shows the corrected field.

Written by the indexing model from the issue text.