Update obsolete tuplesort READTUP() comment.

READTUP() routines do not and cannot use the resettable "tuplecontext"
memory context, since it is deleted when merging begins.  Update an
obsolete comment that claimed otherwise.  This was an oversight in
commit e94568ec.

In passing, fix an unrelated tuplesort typo.

src/backend/utils/sort/tuplesort.c

@@ -574,10 +574,12 @@ struct Sharedsort
  * a lot better than what we were doing before 7.3.  As of 9.6, a
  * separate memory context is used for caller passed tuples.  Resetting
  * it at certain key increments significantly ameliorates fragmentation.
- * Note that this places a responsibility on readtup and copytup routines
- * to use the right memory context for these tuples (and to not use the
- * reset context for anything whose lifetime needs to span multiple
- * external sort runs).
+ * Note that this places a responsibility on copytup routines to use the
+ * correct memory context for these tuples (and to not use the reset
+ * context for anything whose lifetime needs to span multiple external
+ * sort runs).  readtup routines use the slab allocator (they cannot use
+ * the reset context because it gets deleted at the point that merging
+ * begins).
  */
 
 /* When using this macro, beware of double evaluation of len */
@@ -1148,7 +1150,7 @@ tuplesort_begin_datum(Oid datumType, Oid sortOperator, Oid sortCollation,
 	 * Abbreviation is possible here only for by-reference types.  In theory,
 	 * a pass-by-value datatype could have an abbreviated form that is cheaper
 	 * to compare.  In a tuple sort, we could support that, because we can
-	 * always extract the original datum from the tuple is needed.  Here, we
+	 * always extract the original datum from the tuple as needed.  Here, we
 	 * can't, because a datum sort only stores a single copy of the datum; the
 	 * "tuple" field of each sortTuple is NULL.
 	 */
@@ -2821,7 +2823,6 @@ mergeonerun(Tuplesortstate *state)
 		{
 			stup.tupindex = srcTape;
 			tuplesort_heap_replace_top(state, &stup);
-
 		}
 		else
 			tuplesort_heap_delete_top(state);