Update CONTRIBUTING.md

`npx jsii-conf` was failing with `npm ERR! 404 'jsii-conf@latest' is not in the npm registry.`

Author: leothomas

CONTRIBUTING.md

@@ -1,7 +1,7 @@
 # Contributing
 
 1. Create empty package: `npx lerna create @cdk-seed/<package-name>`
-1. Prepare package: `npx jsii-conf -p ./packages/<package-name>/package.json`
+1. Prepare package: `npx jsii-config-p ./packages/<package-name>/package.json`
 
 ## Additional Resources