WebVTT cue setting syntax incorrectly says colon and value are optional

[zcorpan]

b82c0c5 introduced superfluous and incorrect syntax rule for cue settings

  <ol>
    <li>A <a title="WebVTT cue setting name">WebVTT cue setting name</a>.</li>
    <li>An optional U+003A COLON (colon) character.</li>
    <li>An optional <a title="WebVTT cue setting value">WebVTT cue setting value</a>.</li>
  </ol>

Each individual cue setting defines its own syntax, where the colon and value are both required for all defined cue settings.

Either this text should be removed, or fixed to remove the word "optional" for both the colon and the value.