Fix typo in bag_eq() docs

doc/pgtap.mmd

@@ -1560,7 +1560,7 @@ any unexpected overlap between the query results.
 The `bag_eq()` function is just like `set_eq()`, except that it considers the
 results as bags rather than as sets. A bag is a set that allows duplicates. In
 practice, it mean that you can use `bag_eq()` to test result sets where order
-doesn't matter, but duplication does. In other words, if a two rows are the
+doesn't matter, but duplication does. In other words, if two rows are the
 same in the first result set, the same row must appear twice in the second
 result set.