MDCTopAppBar README should not import index file

In the MDCTopAppBar README here.

The import statement should not import the /index but rather the package name.

// From
import {MDCTextField} from '@material/textfield/index';

// To
import {MDCTextField} from '@material/textfield';

As per the importing-js.md and #4509

[kfranqueiro]

That comment is similarly obsolete for the same reason. It was written before we were able to define the module property in package.json (because prior to the TypeScript conversion, our ES modules’ contents were not purely ES5).

[Richienb]

@kfranqueiro Oh. I see.