docs: use proper dialect for redshift (#4823)
docs: use proper dialect for redshift (#4823)

Use redshift+psycopg2 instead of the postgresql one.
dmigo/incubator-superset,apache/incubator-superset,dmigo/incubator-superset,airbnb/caravel,airbnb/superset,mistercrunch/panoramix,zhouyao1994/incubator-superset,airbnb/superset,airbnb/caravel,mistercrunch/panoramix,apache/incubator-superset,timifasubaa/incubator-superset,timifasubaa/incubator-superset,airbnb/superset,apache/incubator-superset,dmigo/incubator-superset,mistercrunch/panoramix,apache/incubator-superset,airbnb/caravel,zhouyao1994/incubator-superset,airbnb/caravel,zhouyao1994/incubator-superset,dmigo/incubator-superset,timifasubaa/incubator-superset,airbnb/superset,mistercrunch/panoramix,zhouyao1994/incubator-superset,apache/incubator-superset,timifasubaa/incubator-superset,zhouyao1994/incubator-superset