name typo in the "spack package management" section of README.md

[JunCEEE]

• What is the issue you have?
  In README.md, the package name in spack is supposed to be nlohmann-json instead of nlohmann_json. Please see this reference: https://spack.readthedocs.io/en/latest/package_list.html#nlohmann-json

• Please describe the steps to reproduce the issue. Can you provide a small but working code example?
  View https://github.com/nlohmann/json#package-managers.

Not a big deal. Thanks a lot for all of your contributions!

[nlohmann]

Thanks! Fixed in bf41560.

[nlohmann]

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🔖 Release item

This issue/PR will be part of the next release of the library. This template helps preparing the release notes.

Type

• ✨ New Feature
• 🐛 Bug Fix
• ⚡️ Improvement
• 🔨 Further Change
• 🔥 Deprecated function

Description

• Fixed name of the spack package.

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[ax3l]

Sorry, that's my fault.