Fix helloworld post dagger core package rename

Follow-up to https://github.com/dagger/dagger/pull/1866

Signed-off-by: Gerhard Lazu <gerhard@lazu.co.uk>

pkg/universe.dagger.io/examples/helloworld/helloworld.cue

@@ -5,17 +5,18 @@
 // 9:06AM INF actions.hello | computing
 // 9:06AM INF actions.hello | #3 0.073 hello, world!
 // 9:06AM INF actions.hello | completed    duration=100ms
-package main
+package helloworld
 
 import (
 	"dagger.io/dagger"
+	"dagger.io/dagger/core"
 )
 
 dagger.#Plan & {
 	actions: {
-		_alpine: dagger.#Pull & {source: "alpine:3"}
+		_alpine: core.#Pull & {source: "alpine:3"}
 		// Hello world
-		hello: dagger.#Exec & {
+		hello: core.#Exec & {
 			input: _alpine.output
 			args: ["echo", "hello, world!"]
 			always: true