Fix typo in mvdistinct.c

Noticed while browsing the code.

src/backend/statistics/mvdistinct.c

@@ -339,7 +339,7 @@ statext_ndistinct_deserialize(bytea *data)
  *		input routine for type pg_ndistinct
  *
  * pg_ndistinct is real enough to be a table column, but it has no
- * operations of its own, and disallows input (jus like pg_node_tree).
+ * operations of its own, and disallows input (just like pg_node_tree).
  */
 Datum
 pg_ndistinct_in(PG_FUNCTION_ARGS)