Change null measurements docstring support from `qml.mutual_info` to `qml.measurements.null_measurement`

[mlxd]

The following file info incorrectly reports the expected content type as qml.mutual_info where it should be qml.measurements.null_measurement.NullMeasurement

pennylane/pennylane/measurements/null_measurement.py

Line 16 in 711375b

This module contains the qml.mutual_info measurement.