docs: import ClientConfig from temporalio.envconfig in README

[shaikn6]

The environment-configuration example in the README imports ClientConfig from temporalio.client and then calls ClientConfig.load_client_connect_config().

temporalio.client.ClientConfig is the TypedDict of client options and has no load_client_connect_config, so the sample fails with AttributeError. The loader lives on temporalio.envconfig.ClientConfig; the sample now imports it from there and keeps Client on the temporalio.client import.

Checked against the released package: from temporalio.envconfig import ClientConfig; ClientConfig.load_client_connect_config exists, temporalio.client.ClientConfig does not have it. Docs-only change.

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