DOCS-503 positional operator

Author: kay-kim

source/reference/operator/positional.txt

@@ -79,5 +79,5 @@
 
    .. seealso::
 
-      :method:`update() <db.collection.update()>`, :operator:`$set`,
+      :method:`update() <db.collection.update()>`, :operator:`$set` and
       :operator:`$unset`