comment: replace cached-routers with cached-descriptors

cached-routers has been gone for a long time

src/feature/nodelist/routerlist.c

@@ -160,7 +160,7 @@ static time_t last_descriptor_download_attempted = 0;
  *
  * From time to time, we replace "cached-descriptors" with a new file
  * containing only the live, non-superseded descriptors, and clear
- * cached-routers.new.
+ * cached-descriptors.new.
  *
  * On startup, we read both files.
  */