Changes twigil to public

There is no reason in this example to show a private attribute.

This clarifies that TWEAK can use $! to access attributes and
that it's not a typo, without going into an explanation at
this stage.

doc/Language/rb-nutshell.pod6

@@ -967,9 +967,9 @@ p = Person.new( name: 'Jack', age: 23 )
 class Person {
     has $.name = 'Jill';
     has $.age  = 42;
-    has $!birth_year;
+    has $.birth_year;
     method TWEAK {
-        $!birth_year = now.Date.year - $.age;
+        $!birth_year = now.Date.year - $!age;
     }
 }
 my $p = Person.new( name => 'Jack', age => 23 )