Mention a trac ticket relevant to an XXX comment.

src/or/circuitbuild.c

@@ -2460,7 +2460,7 @@ pathbias_measure_close_rate(entry_guard_t *guard)
  *
  * XXX: The attempt count transfer stuff here might be done
  * better by keeping separate pending counters that get
- * transfered at circuit close.
+ * transfered at circuit close. See ticket #8160.
  */
 static void
 pathbias_scale_close_rates(entry_guard_t *guard)
@@ -2506,7 +2506,7 @@ pathbias_scale_close_rates(entry_guard_t *guard)
  *
  * XXX: The attempt count transfer stuff here might be done
  * better by keeping separate pending counters that get
- * transfered at circuit close.
+ * transfered at circuit close. See ticket #8160.
  */
 void
 pathbias_scale_use_rates(entry_guard_t *guard)