README.md: themekit node wrapper package name is @shopify/themekit

[lorand-horvath]

Correct npm package name for the Shopify themekit node wrapper is @shopify/themekit

Warn Checklist

• [ ] This changes the interface and requires a Major/Minor version change.
• [ ] I have :tophat:'d these changes by using the commands I changed by hand.
• [ ] I have added a dependancy to the project.
• [ ] I have considered any potential impact on node-themekit