Update README.md

[JohnGriffioen]

The npm run sample_data command was spelt incorrectly. Changed to npm run sample-data

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Governance

• [ ] Documentation is added
• [ ] Test cases are added or updated
• [ ] I've read the contributing document https://github.com/cherrytwist/.github/blob/master/CONTRIBUTING.md
• [ ] I've read and understand the Code of Conduct https://github.com/cherrytwist/.github/blob/master/CODE_OF_CONDUCT.md
• [ ] I understand that any contributions or suggestions I made may make it into the actual code. I've read the License https://github.com/cherrytwist/Coordination/blob/master/LICENSE and the contributor license agreement: tba

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