Fix typo in comment.

Etsuro Fujita

src/backend/commands/matview.c

@@ -473,7 +473,7 @@ transientrel_destroy(DestReceiver *self)
  * the given integer, to make a new table name based on the old one.
  *
  * This leaks memory through palloc(), which won't be cleaned up until the
- * current memory memory context is freed.
+ * current memory context is freed.
  */
 static char *
 make_temptable_name_n(char *tempname, int n)