`first_xray_simulation.ipynp` wrong text for the interactive OpenGL window

[paskino]

The text in the cell describing the interactivity of the OpenGL window:

# Interactive visualisation
# The user can rotate the 3D scene and zoom-in and -out in the visualisation window.

# - Keys are:
#     - Q/Escape: to quit the event loop (does not close the window)
#     - B: display/hide the X-ray beam
#     - W: display the polygon meshes in solid or wireframe
#     - N: display the X-ray image in negative or positive
#     - H: display/hide the X-ray detector
# - Mouse interactions:
#     - Zoom in/out: mouse wheel
#     - Rotation: Right mouse button down + move cursor```

In particular the line # - H: display/hide the X-ray detector should be

#     - D: display/hide the X-ray detector