message: add C++ syntax highlighting
diff: @@ -17,7 +17,7 @@ Previously, the workflow was to create an object, fill it with the necessary
  parameters, and then place the object into an `OSPModel` via, for example,
  `ospAddGeometry`. An example is shown below using the C API.
  -```
  +```cpp
  OSPGeometry mesh = ospNewGeometry("triangles");
  // set parameters on mesh
  ospCommit(mesh);
  @@ -37,7 +37,7 @@ objects.
  The new workflow is shown below. Note that calls to `ospRelease()` have been
  removed for brevity.
  -```
  +```cpp
  // create a geometry
  OSPGeometry mesh = ospNewGeometry("triangles");
  // set parameters on mesh
  