Fixed typo in docs/ref/databases.txt.
@@ -693,9 +693,9 @@ Option MariaDB MySQL
=============== ========= ==========
When using ``select_for_update()`` on MySQL, make sure you filter a queryset
-against at least set of fields contained in unique constraints or only against
-fields covered by indexes. Otherwise, an exclusive write lock will be acquired
-over the full table for the duration of the transaction.
+against at least a set of fields contained in unique constraints or only
+against fields covered by indexes. Otherwise, an exclusive write lock will be
+acquired over the full table for the duration of the transaction.
Automatic typecasting can cause unexpected results
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