Fix a comment.

src/Clock.ml

@@ -24,7 +24,7 @@ type clock = {
   mutable now: float;
   (* The number of ticks that have taken place. *)
   mutable ticks: int;
-  (* The last time [tick] was called for this clock. *)
+  (* The last time a user function [f] was called via [tick clock f]. *)
   mutable last: float;
   (* A circular array of the times at which the most recent round ticks
      took place. *)