Update sycl/doc/extensions/experimental/sycl_ext_oneapi_graph.asciidoc

Author: reble

sycl/doc/extensions/experimental/sycl_ext_oneapi_graph.asciidoc

@@ -1026,7 +1026,7 @@ object.
 [source,c++]
 ----
 command_graph<graph_state::modifiable>
-queue::ext_oneapi_get_graph();
+queue::ext_oneapi_get_graph() const;
 ----
 
 | Query the underlying command graph of a queue when recording.