Add a missing invocant to the method definition

Author: book

doc/Type/Metamodel/ClassHOW.pod6

@@ -38,7 +38,7 @@ C<Metamodel::ClassHOW> is the meta class behind the C<class> keyword.
 
 =head2 method add_fallback
 
-    method add_fallback(Metamodel::ClassHOW:D: $condition, $calculator)
+    method add_fallback(Metamodel::ClassHOW:D: $obj, $condition, $calculator)
 
 Installs a method fallback, that is, add a way to call methods that weren't
 statically added.