Simplify the top-level section titles

Repeating "WebVTT" everywhere isn't very helpful.

These names are used in web-platform-test, where it look rather silly:
https://github.com/w3c/web-platform-tests/tree/d6b71427f5dbfa0588baf1de5634f6016baff103/webvtt
Merge pull request #58 from foolip/section-titles

Simplify the top-level section titles