update typings entry point in npm's package.json

https://github.com/angular/angular/blob/4f786f2c6766f7413d359f2170ddedbe0aa492a2/package.json#L16

@alexeagle suggested that this should point to `core.d.ts` instead with the side effect being that `import ... from 'angular2/core'` and `import ... from 'angular2'` would be the same thing.
