Docs - Add 'run' to npm install-all command in README

[IanFindlay]

Proposed changes

When setting up this project to work on a different issue, I followed the README and ran in to problems with the npm install-all command. As it is a custom command, I am pretty sure it has to be npm run install-all as this is what worked for me.

Types of changes

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• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [x] Documentation update (Documentation content changed)
• [ ] Other (please describe):

Checklist

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• [x] My code follows the style guidelines of this project
• [x] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [x] I have made corresponding changes to the documentation
• [x] My changes generate no new warnings