Fixed a bug that meant redis database numbers greater than could not be used.

For example
If you give the Predis connector a connection dsn similar to 'redis://127.0.0.1:6379/3',
the expected behaviour is to use redis database 3, but it doesn't.
It always uses redis db 0.

The fix was to correct a small typo in the Rhubarb/Connector/Predis file.