DOC: correct description of parameter sigma_t (std of the metric value ACROSS labels)
DOC: correct description of parameter sigma_t (std of the metric value ACROSS labels)

3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox