Use a more explicit CSS selector example.

Since WebVTT is based on SRT and SRT has numbers as identifiers for
cues, the selection of cues by number is a common use case and worth an
example.

Closes https://www.w3.org/Bugs/Public/show_bug.cgi?id=19632
Merge pull request #43 from silviapfeiffer/css_example

Use a more explicit CSS selector example.