fix example (thanks to Michael Stevens)

[SVN r11979]

generator_iterator.htm

@@ -135,7 +135,7 @@ private:
 int main()
 {
   my_generator gen;
-  boost::generator_iterator<my_generator> it(gen);
+  boost::generator_iterator_generator<my_generator>::type it = boost::make_generator_iterator(gen);
   for(int i = 0; i < 10; ++i, ++it)
     std::cout << *it << std::endl;
 }