Changes main path in packages.json

[kareemsalah227]

Problem:

When fetching the package using npm install in a create-react-app, user will get a compilation error that ra-data-springboot-data can not be resolved.

The problem is because the main entry defined in packages.json is pointing to a non-existing path lib/index.js.

Solution:

Either create the wrong path and copy index.js under the created path lib, or just change the main entry to src/index.js, I did the latter.

Reference:

#4

@vishpat I'll mention you since I can't add you as a reviewer.

[jbournonville]

@vishpat did you have the possibility to merge this PR ?