Update grid auto placement example

[OnkarRuikar]

• fixes https://github.com/mdn/css-examples/issues/66
• fixes https://github.com/mdn/content/issues/13721

Problem

There are no gaps in the grid without using dense algorithm. So removing the grid-auto-flow: dense line has no effect.

Solution

Interleaving big and small blocks is not helping here. Putting bigger blocks together creates noticeable amount of gaps.