ARROW-4089: [Plasma] The tutorial is wrong regarding the parameter type of PlasmaClient.Create

[sarutak]

Plasma's tutorial says that the data type of one parameter is address of uint8_t* but it's actually address of shared_ptr<Buffer>.

uint8_t* data;                <------------------------------- wrong data type here.
 // Create a Plasma object by specifying its ID and size.
 ARROW_CHECK_OK(client.Create(object_id, data_size, NULL, 0, &data));

[wesm]

We should perhaps turn this into an example we compile and include it in the Sphinx documentation