potential fix for bug on webpage example under 3.2.2

In the 2nd code section of this area (starting with `dataset = nilearn.datasets.fetch_adhd()`, in the loop `n_subjects` is referenced, but doesn't appear in that code area.  I see in the `doc/extracting_data.rst` file that this code references the file `plot_adhd_covariance.py`, which has the `n_subjects` referenced at the top of the file.  So I just moved the `n_subjects` reference to appear in that code section.  Not sure if this is where it should be referenced, but this could certainly cause confusion for folks trying to go through the example.