[1.10.x] Fixed #27209 -- Fixed typo in docs/ref/models/database-functions.txt.

Thanks Valentin Ignatyev for the report.

Backport of 767ba009764c6640b20ea6a6dc6ee2559cc87800 from master

docs/ref/models/database-functions.txt

@@ -37,7 +37,7 @@ Usage example::
     >>> from django.db.models import FloatField
     >>> from django.db.models.functions import Cast
     >>> Value.objects.create(integer=4)
-    >>> value = Value.objects.annotate(as_float=Cast('integer', FloatField)).get()
+    >>> value = Value.objects.annotate(as_float=Cast('integer', FloatField())).get()
     >>> print(value.as_float)
     4.0