Fix documentation for adding a Redshift database (#2447)

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](#2364 (comment))).

The fix is to use `postgresql+psycog2://...` for the Redshift connection
string.

docs/installation.rst

@@ -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://``                                    |
 +---------------+-------------------------------------+-------------------------------------------------+