Remove extraneous space between last two items.

There is an extra space between the last two items, so the output of `coauthors()` would generate the output `John Smith, John Doe  and Jane Doe`. It was really noticeable when I tried adding a serial comma with `coauthors( null, ', and ' )`. The output was `John Smith, John Doe , and Jane Doe` (notice the extra space between John Doe and the comma).