fix RST

Author: dunglas

reference/constraints/File.rst

@@ -265,7 +265,7 @@ You can find a list of existing mime types on the `IANA website`_.
     the form builder) and when the field doesn't define its own ``accept`` value.
 
 ``extensionsMessage``
-~~~~~~~~~~~~~~~~~~~~
+~~~~~~~~~~~~~~~~~~~~~
 
 **type**: ``string`` **default**: ``The extension of the file is invalid ({{ extension }}). Allowed extensions are {{ extensions }}.``