using constructor for command graph creation (I/II)

Co-authored-by: Ronan Keryell <ronan@keryell.fr>

Author: reble

sycl/doc/extensions/proposed/sycl_ext_oneapi_graph.asciidoc

@@ -371,7 +371,7 @@ int main() {
 
   sycl::queue q;
 
-  auto g = sycl::ext::oneapi::experimental::make_graph();
+  sycl::ext::oneapi::experimental::command_graph g;
 
   float *x , *y, *z;