Removed the unnecessary *= -1 from Spectral Timing Exploration.ipynb

[pupperemeritus]

Tried to fix the issue StingraySoftware/stingray#691. Please provide feedback. One change that happened in the time lag - frequency graph has flipped from the result in the documentation. I am not sure whether it is occuring due to an intended change or I messed up something. The large deletion count is due to the progress bars rendering dynamically in one line as opposed to one line for each percentage.

[matteobachetti]

Thanks @pupperemeritus
Now that you're at it, the string f"Time lag ({ref_band[0]:g}-{ref_band[1]:g} keV vs {sub_band[0]:g}-{sub_band[1]:g} keV, in seconds)" should be changed, inverting ref_band and sub_band

[pupperemeritus]

Changed it

[matteobachetti]

Thanks @pupperemeritus!