Measurements Block: minor bug in documentation

[aslgraefe]

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

[pnrobinson]

The protobuf code is a "oneof"

 oneof measurement_value {
    Value value = 3;
    ComplexValue complex_value = 4;
  }

In the resulting phenopacket, we would not actually see "measurement_value", but instead either "value" or "complex_value".

@julesjacobsen should we add a note to the documentation about this? I am worried it might be too much detail...