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

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));
```

Author: Kousuke Saruta <sarutak@oss.nttdata.com>

Closes #3235 from sarutak/fix-plasma-tutorial and squashes the following commits:

a780a27cf <Kousuke Saruta> Fix the data type of the pointer in the plasma's tutorial