[css-timing-1] Drop mention of before flag as input to frames timing function

Author: birtles

css-timing/Overview.bs

@@ -361,7 +361,7 @@ As with [=step timing functions=], at the exact point where a step occurs the
 result of the function is conceptually the top of the step.
 
 The [=output progress value=] is calculated from the [=input progress value=]
-and [=before flag=] as follows:
+as follows:
 
 1.   Calculate the <var>current frame</var> as <code>floor([=input progress
      value=] &times; [=frames=])</code>.