Fix the definitions for fill forwards/backwards (#38)

Although correct in the currentTime algorithm, the definitions were
inverted. Swap them with each other so that the definition matches what
they actually mean.

Author: stephenmcgruer

Overview.bs

@@ -460,31 +460,31 @@ not by wall-clock time, but by the progress of scrolling in a [=scroll container
 
     :   none
     ::  The timeline is inactive when the scroll offset is less than {{startScrollOffset}} 
-        or greater than {{endScrollOffset}}.
+        or greater than or equal to {{endScrollOffset}}.
 
     :   forwards
     ::  When the scroll offset is less than {{startScrollOffset}}, the
-        timeline's [=current time=] is 0.
-        When the scroll offset is greater than {{endScrollOffset}}, the
         timeline is inactive.
+        When the scroll offset is greater than or equal to the
+        {{endScrollOffset}}, the timeline's [=current time=] is its
+        [=effective time range=].
 
     :   backwards
     ::  When the scroll offset is less than {{startScrollOffset}}, the
-        timeline is inactive.
-        When the scroll offset is greater than {{endScrollOffset}}, the
-        timeline's [=current time=] is its 
-        [=effective time range=].
+        timeline's [=current time=] is 0.
+        When the scroll offset is greater than or equal to the
+        {{endScrollOffset}}, the timeline is inactive.
 
     :   both
     ::  When the scroll offset is less than {{startScrollOffset}}, the
         timeline's [=current time=] is 0.
-        When the scroll offset is greater than {{endScrollOffset}}, the
-        timeline's [=current time=] is its 
+        When the scroll offset is greater than or equal to the
+        {{endScrollOffset}}, the timeline's [=current time=] is its
         [=effective time range=].
 
     :   auto
     ::  Behaves the same as both.
-        
+
 
 
 ### The effective time range of a {{ScrollTimeline}} ### {#efffective-time-range-algorithm}