Fix typo in #development section

[relisora]

When trying to contribute, I noticed that there was run missing in npm run build command. This fixes the readme.

Types of changes

• Bug fix (a non-breaking change which fixes an issue)
• New feature (a non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Description

npm build -> npm run build

Fixes building command for npm

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes (if not applicable, please state why)

[netlify]

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🔨 Latest commit	60d0e5e

[larbish]

thanks!