Update header block class on snippets page

## What does this MR do?
Changes gray block in `Snippets` to new white sub header style

## What are the relevant issue numbers?
Closes #19692 

## Screenshots (if relevant)
![Screen_Shot_2016-07-11_at_4.17.10_PM](/uploads/f4cea2d019d30fc7c6adc7e3b26590b4/Screen_Shot_2016-07-11_at_4.17.10_PM.png)

See merge request !5192