message: docs: use proper dialect for redshift
  Use redshift+psycopg2 instead of the postgresql one.
diff: @@ -298,7 +298,7 @@ Here's a list of some of the recommended packages.
  +---------------+-------------------------------------+-------------------------------------------------+
  | sqlite | | ``sqlite://`` |
  +---------------+-------------------------------------+-------------------------------------------------+
  -| Redshift | ``pip install sqlalchemy-redshift`` | ``postgresql+psycopg2://`` |
  +| Redshift | ``pip install sqlalchemy-redshift`` | ``redshift+psycopg2://`` |
  +---------------+-------------------------------------+-------------------------------------------------+
  | MSSQL | ``pip install pymssql`` | ``mssql://`` |
  +---------------+-------------------------------------+-------------------------------------------------+
  