Fix the "greatest" example.

Fix the "greatest" example.

The current example code does not pass `**kw` down to the `compiler.process` calls, thus the example does not work when invoked with, `literal_binds=True`.

Besides, the calls to `process` each argument twice are wasteful, and reusing the built-in `case` expression instead of hard-coding the SQL statements is slightly nicer overall, isn't it?

Closes: #4402
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/4402
Pull-request-sha: 24ee93f63e21fccae6cbc1cc1c154dd56f1e1963

Change-Id: I02424d9eb2b35abd5cdec5c2cd5d464a56e7fae6