doc: tests: Remove ref. to undefined sym. CONFIG_ADC_QMSI_SAMPLE_WIDTH

CONFIG_ADC_QMSI_SAMPLE_WIDTH was removed in commit acc40d99 ("adc:
remove adc_qmsi driver"). It's only used as an example in the
documentation, but might as well use an existing symbol instead.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>