docs: fix voltage unit

[myd7349]

The voltage unit used in Example 2 appears to be incorrect.

In the code, the unit is set to microvolts (uV), while the variable name and printed label indicate millivolts (mV), leading to an inconsistency:

from mffpy import Reader
fo = Reader("./examples/example_1.mff")
fo.set_unit('EEG', 'uV')
eeg_in_mV, t0_EEG = fo.get_physical_samples_from_epoch(fo.epochs[0], dt=0.1)['EEG']
fo.set_unit('EEG', 'V')
eeg_in_V, t0_EEG = fo.get_physical_samples_from_epoch(fo.epochs[0], dt=0.1)['EEG']
print('data in mV:', eeg_in_mV[0])
print('data in V :', eeg_in_V[0])

This PR resolves the inconsistency by aligning the unit, variable names, and output labels.

An alternative fix would be to explicitly use millivolts (mV) instead:

from mffpy import Reader
fo = Reader("./examples/example_1.mff")
fo.set_unit('EEG', 'mV')
eeg_in_mV, t0_EEG = fo.get_physical_samples_from_epoch(fo.epochs[0], dt=0.1)['EEG']
fo.set_unit('EEG', 'V')
eeg_in_V, t0_EEG = fo.get_physical_samples_from_epoch(fo.epochs[0], dt=0.1)['EEG']
print('data in mV:', eeg_in_mV[0])
print('data in V :', eeg_in_V[0])

However, since microvolts (uV) are more commonly used in EEG applications than millivolts (mV), the former approach was chosen.

Another possible option would be to include all three units (uV, mV, and V) in the example for completeness.