[web-animations-1] Fix typos in getKeyframes() processing section

This fixes w3c#2174.

Author: birtles

web-animations-1/Overview.bs

@@ -4690,7 +4690,7 @@ interface KeyframeEffect : KeyframeEffectReadOnly {
             1.  Let <var>property name</var> be the result of applying the
                 <a>animation property name to IDL attribute name</a> algorithm
                 to the property name of <var>declaration</var>.
-            1.  Let <var>IDL value</var> be result of serializing the
+            1.  Let <var>IDL value</var> be the result of serializing
                 the property value of <var>declaration</var> by passing
                 <var>declaration</var> to the algorithm to <a>serialize a CSS
                 value</a> [[!CSSOM]].