Fix some grammatical errors in some comments

Introduced by 9f1337639.

Author: James Coleman <jtc331@gmail.com>
Discussion: https://postgr.es/m/CAAaqYe9ZQ_1+QiF_Nv7b37opicBu+35ZQK1CetQ54r5UdrF1eg@mail.gmail.com

src/backend/optimizer/plan/subselect.c

@@ -1271,15 +1271,15 @@ convert_ANY_sublink_to_join(PlannerInfo *root, SubLink *sublink,
 	Assert(sublink->subLinkType == ANY_SUBLINK);
 
 	/*
-	 * If the sub-select refers to any Vars of the parent query, we so let's
-	 * considering it as LATERAL.  (Vars of higher levels don't matter here.)
+	 * If the sub-select contains any Vars of the parent query, we treat it as
+	 * LATERAL.  (Vars from higher levels don't matter here.)
 	 */
 	sub_ref_outer_relids = pull_varnos_of_level(NULL, (Node *) subselect, 1);
 	use_lateral = !bms_is_empty(sub_ref_outer_relids);
 
 	/*
-	 * Check that sub-select refers nothing outside of available_rels of the
-	 * parent query.
+	 * Can't convert if the sub-select contains parent-level Vars of relations
+	 * not in available_rels.
 	 */
 	if (!bms_is_subset(sub_ref_outer_relids, available_rels))
 		return NULL;