[web-animations] Typo in the keyframe spacing modes definitions

[Aleksei Semenov]

Hello, everyone.

https://w3c.github.io/web-animations/#distribute-keyframe-spacing-mode  
reads:
distribute
 Indicates that keyframes with a null keyframe offset null are positioned 
 so that the difference between subsequent keyframe offsets are equal.

I believe second word 'null' is redundant.

The same problem is here  
https://w3c.github.io/web-animations/#paced-keyframe-spacing-mode
paced

 Indicates that keyframes with a null keyframe offset null are positioned 
 so that the distance between subsequent values of a specified paced  
property are equal.

Thanks,
Aleksei.