Clarify python-order-utils README usage example

[TheAlonze01]

Make the README usage example copy-pastable by:

• importing OrderData explicitly,
• reading EXCHANGE_ADDRESS, CHAIN_ID and PRIVATE_KEY from environment variables.

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Note

Updates README usage example to read EXCHANGE_ADDRESS, CHAIN_ID, and PRIVATE_KEY from env vars and import OrderData (adding os import).

• Docs (README.md):
  • Usage example now reads EXCHANGE_ADDRESS, CHAIN_ID, and PRIVATE_KEY from environment variables via os.environ (casts CHAIN_ID to int).
  • Adds explicit import of OrderData and os.

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