vm_object: Make a comment more clear

Reviewed by:	alc, kib
MFC after:	2 weeks
Differential Revision:	https://reviews.freebsd.org/D49675

sys/vm/vm_object.c

@@ -1603,7 +1603,7 @@ retry:
 		 * One other possibility is that the map entry is wired, in
 		 * which case we must hang on to the page to avoid leaking it,
 		 * as the map entry owns the wiring.  This case can arise if the
-		 * backing pager is truncated.
+		 * backing object is truncated by the pager.
 		 */
 		if (vm_page_none_valid(m) && entry->wired_count == 0) {
 			if (vm_page_iter_remove(&pages, m))