cwt: log scales in docstring example

[OverLordGoldDragon]

I saw linear scales in some important code recently (written by those who don't know better).

There will almost never be a use case for this. CWT is a log transform and should be tiled logarithmically.

If my example of an exponential chirp is unsatisfying, feel free to revert, but it's what illustrates the flaw of the current approach; the signal should trace out a straight line in CWT.

[rgommers]

Thanks @OverLordGoldDragon, this looks like a useful change to make. In the rest of the narrative docs for cwt (https://pywavelets.readthedocs.io/en/latest/ref/cwt.html) it's emphasized multiple times that it should be log scale, so making it consistent also in the docstring examples makes sense.