Fix typo in keyframe spacing definition

As reported:
https://lists.w3.org/Archives/Public/public-fx/2015JanMar/0089.html

Overview.src.html

@@ -4075,11 +4075,11 @@ <h4 id="spacing-keyframes">Spacing keyframes</h4>
 
 :   <dfn lt="distribute keyframe spacing mode">distribute</dfn></dt>
 ::  Indicates that <a>keyframes</a> with a null <a>keyframe offset</a>
-    null are positioned so that the difference between subsequent
+    are positioned so that the difference between subsequent
     <a>keyframe offsets</a> are equal.
 :   <dfn lt="paced keyframe spacing mode">paced</dfn></dt>
 ::  Indicates that <a>keyframes</a> with a null <a>keyframe offset</a>
-    null are positioned so that the <em>distance</em> between subsequent
+    are positioned so that the <em>distance</em> between subsequent
     values of a specified <dfn>paced property</dfn> are equal.
     The distance is calculated using the <a>distance computation</a>
     procedure defined by the <a>animation behavior</a> associated