[web-animations] Paced spacing mode without paced property is still used

Hi All,

Example 6, Example 7 and Figures 28 and 29 in non-normative section of
4.3.1. Spacing keyframes
(http://w3c.github.io/web-animations/#spacing-keyframes) still use "paced"
spacing mode without paced property specified. According the latest
changes in the spec paced property is required.

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Regards,
Sergey G. Grekhov