Small tweaks to un-bitrot classtut's first example.

Some of these tweaks may change the message that the example is
sending, so this commit is mostly to show a changeset to the fine
people of #perl6 for feedback.

lib/Language/classtut.pod

@@ -13,11 +13,11 @@ result is interesting and, at times, useful.
 
     =begin code
     class Task {
-        has      &!callback;
-        has Task @!dependencies;
+        has      &.callback;
+        has Task @.dependencies;
         has Bool $.done;
 
-        method new(&callback, Task *@dependencies) {
+        method new(&callback, *@dependencies) {
             return self.bless(:&callback, :@dependencies);
         }
 
@@ -27,8 +27,8 @@ result is interesting and, at times, useful.
 
         method perform() {
             unless $!done {
-                .perform() for @!dependencies;
-                &!callback();
+                .perform() for @.dependencies;
+                &.callback.();
                 $!done = True;
             }
         }