[css-lists-3] Correct the counter-set example

[OnkarRuikar]

For following reasons I think the example has a typo and it should use counter-set instead of counter-reset:

• the example is part of the Manipulating Counter Values: the counter-increment and counter-set properties section
• counter-reset creates a new counter so the comment /* Set section to 0 */ is not accurate. The counter-set makes the comment more accurate.