Improve spherize documentation

[gwaybio]

The spherize notebook documentation should be improved.

@shntnu notes in #60

The notebook says

Here, we load in all normalized profiles (level 4a) data across all plates and apply a spherize transform using the DMSO profiles as the background distribution.

but it should say

Here, we load in all normalized profiles (level 4a) data across all plates, apply the standard set of feature selection operations, and then apply a spherize transform using the DMSO profiles as the background distribution.

This is a very easy fix, and a good beginner issue!