Correct typo in spectral analysis Notebook

[tomarney]

The call to interp() in the Spectral Analysis notebook ("Effect of Regridding" section) uses the Series.value to calculate min and max step, but it should be the Sries.time property.

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[CommonClimate]

Hi @tomarney. Good catch and thank you for making this PR, which I only just noticed.
I'm surprised the results appear nearly unchanged before/after the change, though. Either something is very fishy, or there is a miraculous conspiring of numbers. In any case, I realized when reviewing your work that the better way to do this was to use the Resolution class, so I added that to your PR, and will now close it.