message: Fix documentation for adding a Redshift database
  When adding a Redshift database, the docs said to use a connection
  string that started with `redshift+psycopg2://...`, but this results
  in a Python error from how SQLAlchemy executes the `get_schema_names`
  method (first reported [here](https://github.com/airbnb/superset/issues/2364#issuecomment-284705179)).
  The fix is to use `postgresql+psycog2://...` for the Redshift connection
  string.
diff: @@ -196,7 +196,7 @@ Here's a list of some of the recommended packages.
  +---------------+-------------------------------------+-------------------------------------------------+
  | sqlite | | ``sqlite://`` |
  +---------------+-------------------------------------+-------------------------------------------------+
  -| Redshift | ``pip install sqlalchemy-redshift`` | ``redshift+psycopg2://`` |
  +| Redshift | ``pip install sqlalchemy-redshift`` | ``postgresql+psycopg2://`` |
  +---------------+-------------------------------------+-------------------------------------------------+
  | MSSQL | ``pip install pymssql`` | ``mssql://`` |
  +---------------+-------------------------------------+-------------------------------------------------+
  