[css-easing-2] typo

Author: tabatkins

css-easing-2/Overview.bs

@@ -260,7 +260,7 @@ Serializing</h4>
     return |s|.
 
   3. Otherwise, append " " (U+0020 SPACE) to |s|,
-    then serialize the [=control point's=] [=output progress value=]
+    then serialize the [=control point's=] [=input progress value=]
     as a <<percentage>>
     and append it to |s|.