README client should call fga_client instead of api_instance

[adriantam]

Currently, the (README file)[https://github.com/openfga/python-sdk/blob/main/README.md] is incorrect in several places where api_instance is used instead of fga_client.

The sample code for

• https://github.com/openfga/python-sdk#relationship-tuples
• https://github.com/openfga/python-sdk#read-relationship-tuples
• https://github.com/openfga/python-sdk#list-objects

uses api_instance instead of fga_client. The purpose of this issue is to change all these instances to use fga_client instead.

Ideally, we will also fix the SDK generator so that newly generated python SDKs will have the corresponding changes. This is tracked as openfga/sdk-generator#126. However, if that is difficult to do, simply focusing on the Python SDK side https://github.com/openfga/python-sdk/blob/main/README.md will help us as well.

[Harshal662]

i can do that