[css-easing-1] Fix references to step numbers

Author: birtles

css-easing-1/Overview.bs

@@ -373,7 +373,7 @@ and [=before flag=] as follows:
 
      <div class=note>
 
-     Steps 5 and 6 in this procedure ensure that given an [=input progress
+     Steps 4 and 6 in this procedure ensure that given an [=input progress
      value=] in the range [0, 1], a step timing function does not produce an
      [=output progress value=] outside that range.