Changes BUILD to TWEAK in code example

Code example now works using TWEAK instead of BUILD.

Changes description to mentions TWEAK instead of BUILD too.

Author: MorayJ

doc/Language/rb-nutshell.pod6

@@ -945,7 +945,7 @@ class Person {
 Creating a new instance of the class uses the C<.new> method. In Ruby you must
 manually assign instance variables as needed inside C<initialize>. In Raku
 you get a default constructor that accepts key/value pairs of accessor
-attributes, and can do further setup in the C<BUILD> method. Like with Ruby,
+attributes, and can do further setup in the C<TWEAK> method. Like with Ruby,
 you can override C<new> itself for more advanced functionality, but this is
 rare.
 
@@ -968,7 +968,7 @@ class Person {
     has $.name = 'Jill';
     has $.age  = 42;
     has $!birth_year;
-    method BUILD {
+    method TWEAK {
         $!birth_year = now.Date.year - $.age;
     }
 }