chore: convert hello_world to v3

examples/hello_world/validators/hello_world.ak

@@ -3,16 +3,16 @@ use aiken/list
 use aiken/transaction.{ScriptContext}
 use aiken/transaction/credential.{VerificationKey}
 
-type Datum {
+pub type Datum {
   owner: Hash<Blake2b_224, VerificationKey>,
 }
 
-type Redeemer {
+pub type Redeemer {
   msg: ByteArray,
 }
 
-validator {
-  fn spend(datum: Datum, redeemer: Redeemer, context: ScriptContext) -> Bool {
+validator hello_world {
+  spend(datum: Datum, redeemer: Redeemer, context: ScriptContext) -> Bool {
     let must_say_hello = redeemer.msg == "Hello, World!"
 
     let must_be_signed =
@@ -20,4 +20,8 @@ validator {
 
     must_say_hello && must_be_signed
   }
+
+  else(_ctx) {
+    fail
+  }
 }