[css-lists-3] Correct markup for nested lists example

[manuel-za]

Non-substantive contribution.

Before:

• Markup for css-lists-3/Example 19 (nested lists) does not render the expected layout
• The body of the line item "Third second-level item, list start=3" indicates the intention of the authors

After:

• Minor correction to the markup matches likely author intent

[w3cbot]

SebastianZ marked as non substantive for IPR from ash-nazg.

[SebastianZ]

Thanks for the fix, @manuel-za!

Sebastian