[docs] Fix description for "Striped rows" example

[lindapaiste]

This PR fixes a mistake introduced in #3882

Mistake

The description for the "Custom theme" example contains text that is meant for the "Striped rows" example:

While the "Striped rows" example description has a typo ("stripped" instead of "striped"):

Source

The mistake happened in this commit.

Fix

• Re-applied the previous (pre-[DataGrid] Add indexes relative to the filtered rows and the current page to the getRowClassName and getRowSpacing props #3882) description for the "Custom theme" example.
• Moved the current "Custom theme" description to "Striped rows".

[mui-bot]

These are the results for the performance tests:

Test case	Unit	Min	Max	Median	Mean	σ
Filter 100k rows	ms	243.8	404.4	317.8	335.4	61.382
Sort 100k rows	ms	444.7	929	713.9	698.7	162.081
Select 100k rows	ms	144.4	200.2	178.6	174.56	19.63
Deselect 100k rows	ms	109.7	280.1	201.6	182.88	63.956

Generated by 🚫 dangerJS against 753fad3

[m4theushw]

Thanks!