[1.5.X] Fixed typo in docs/ref/models/querysets.txt

Usually, using ``select_related()`` can vastly improve performance because your

app can avoid many database calls. However, there are times you are only

interested in specific related models, or have deeply nested sets of

relationships, and in these cases ``select_related()`` can can be optimized by

relationships, and in these cases ``select_related()`` can be optimized by

explicitly passing the related field names you are interested in. Only

the specified relations will be followed.