[web-animations-1] Typos in getKeyframes() processing section.

[graouts]

There's both a missing and a duplicated "the" in the getKeyframes() section, specifically step 3.1.4.2:

Let IDL value be result of serializing the the property value of declaration by passing declaration to the algorithm to serialize a CSS value [CSSOM].

This should be:

Let IDL value be the result of serializing the property value of declaration by passing declaration to the algorithm to serialize a CSS value [CSSOM].