Fix text excerpts sample for HTML / CSS example

[5im0n]

It's possible to create text excerpts with multiple line only in CSS. No JavaScript is needed.

Presubmit checklist

• [X] This PR doesn’t contain automatically generated corrections (Grammarly or similar).
• [X] This PR follows the Google Developer Documentation Style Guidelines — for example, it doesn’t use i.e. or e.g., and it avoids I and we (first person).
• [X] This PR uses semantic line breaks of 80 characters or fewer.

[khanhnwin]

Once the indentation is fixed, LGTM!