doc: IMMUTANT_HOME needs to point to `immutant` dir, not `stage`
doc: IMMUTANT_HOME needs to point to `immutant` dir, not `stage`

immutant/immutant,immutant/immutant,kbaribeau/immutant,immutant/immutant,kbaribeau/immutant,kbaribeau/immutant,immutant/immutant,coopsource/immutant,coopsource/immutant,coopsource/immutant