DOC: don't use single letter variable name in _compute_forward (#8727)
DOC: don't use single letter variable name in _compute_forward (#8727)

* ENH: don't use single letter variable name

* Pep8
wmvanvliet/mne-python,Teekuningas/mne-python,larsoner/mne-python,rkmaddox/mne-python,kingjr/mne-python,bloyl/mne-python,pravsripad/mne-python,kingjr/mne-python,drammock/mne-python,olafhauk/mne-python,kingjr/mne-python,wmvanvliet/mne-python,Teekuningas/mne-python,Eric89GXL/mne-python,mne-tools/mne-python,pravsripad/mne-python,drammock/mne-python,mne-tools/mne-python,kambysese/mne-python,bloyl/mne-python,olafhauk/mne-python,Eric89GXL/mne-python,drammock/mne-python,pravsripad/mne-python,larsoner/mne-python,rkmaddox/mne-python,Teekuningas/mne-python,kambysese/mne-python,mne-tools/mne-python,wmvanvliet/mne-python,larsoner/mne-python,olafhauk/mne-python