Update `core/README`

[SeiyaKobayashi]

📝 Overview

Update README of core package to fix typos in https://github.com/jcam1/python-sdk/tree/main/packages/core#1-install-jpyc_core_sdk-package.

🖋 Details

Package name should be jpyc-core-sdk, not jpyc_core_sdk.

🍭 Sub-Tasks

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📚 References

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