Add missing import to tutorial for `lesson03`

[ackerleytng]

Here's a missing import that would help newbies!

(Should be added at the #add-some-imports section for lesson03 for the Formatter and Publisher classes)

import javax.ws.rs.core.Context;

And some imports for the Hello class:

import io.opentracing.tag.Tags;
import io.opentracing.propagation.Format;