Fix c++2b errors in add_phoenix.cpp (op[] can take multiple arguments now)

example/add_phoenix.cpp

@@ -24,7 +24,7 @@ int main(void) {
     // Passed to template, `factor` by constant, and defined in expression.
     std::for_each(nums, nums + 3, let(_f = cref(factor))[
         // Unfortunately, body cannot use C++ statement syntax.
-        ref(sum) += _f * _1, _1 // Access `sum` by reference.
+        ( ref(sum) += _f * _1, _1 ) // Access `sum` by reference.
     ]);
 
     BOOST_TEST(sum == 60);