Fixed #20511 -- Corrected link about isolation levels in databases docs

Thanks tinodb for the report.

docs/ref/databases.txt

@@ -141,10 +141,9 @@ Isolation level
 .. versionadded:: 1.6
 
 Like PostgreSQL itself, Django defaults to the ``READ COMMITTED`` `isolation
-level <postgresql-isolation-levels>`_. If you need a higher isolation level
-such as ``REPEATABLE READ`` or ``SERIALIZABLE``, set it in the
-:setting:`OPTIONS` part of your database configuration in
-:setting:`DATABASES`::
+level`_. If you need a higher isolation level such as ``REPEATABLE READ`` or
+``SERIALIZABLE``, set it in the :setting:`OPTIONS` part of your database
+configuration in :setting:`DATABASES`::
 
     import psycopg2.extensions
 
@@ -161,7 +160,7 @@ such as ``REPEATABLE READ`` or ``SERIALIZABLE``, set it in the
     handle exceptions raised on serialization failures. This option is
     designed for advanced uses.
 
-.. _postgresql-isolation-levels: http://www.postgresql.org/docs/current/static/transaction-iso.html
+.. _isolation level: http://www.postgresql.org/docs/current/static/transaction-iso.html
 
 Indexes for ``varchar`` and ``text`` columns
 --------------------------------------------