Fixed heading level typo in docs/ref/contrib/postgres/fields.txt.
@@ -881,7 +881,7 @@ Returned objects are empty ranges. Can be chained to valid lookups for a
<QuerySet []>
Defining your own range types
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+-----------------------------
PostgreSQL allows the definition of custom range types. Django's model and form
field implementations use base classes below, and psycopg2 provides a
