ACPI: docs: enumeration: Correct reference to the I²C device data type

I²C peripheral devices that are connected to the controller are
represented in the Linux kernel as objects of the struct i2c_client.
Fix this in the documentation.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rjw@rjwysocki.net>