Fix typos in comments about extended statistics

Reported-by: Justin Pryzby
Discussion: https://postgr.es/m/20210505210947.GA27406%40telsasoft.com

src/backend/parser/parse_utilcmd.c

@@ -1945,7 +1945,7 @@ generateClonedExtStatsStmt(RangeVar *heapRel, Oid heapRelid,
 	 * simply append them after simple column references.
 	 *
 	 * XXX Some places during build/estimation treat expressions as if they
-	 * are before atttibutes, but for the CREATE command that's entirely
+	 * are before attributes, but for the CREATE command that's entirely
 	 * irrelevant.
 	 */
 	datum = SysCacheGetAttr(STATEXTOID, ht_stats,

src/backend/statistics/extended_stats.c

@@ -1796,7 +1796,7 @@ statext_mcv_clauselist_selectivity(PlannerInfo *root, List *clauses, int varReli
 				continue;
 
 			/*
-			 * Now we know the clause is compatible (we have either atttnums
+			 * Now we know the clause is compatible (we have either attnums
 			 * or expressions extracted from it), and was not estimated yet.
 			 */