Fixed a reference

Author: javiereguiluz

reference/constraints/File.rst

@@ -248,7 +248,7 @@ You can find a list of existing mime types on the `IANA website`_.
     the value of the ``mimeTypes`` option is also used in the ``accept``
     attribute of the related ``<input type="widget"/>`` HTML element.
 
-    This behavior is applied only when using :ref:`form type guessing <forms-field-guessing>`
+    This behavior is applied only when using :ref:`form type guessing <form-type-guessing>`
     (i.e. the form type is not defined explicitly in the ``->add()`` method of
     the form builder) and when the field doesn't define its own ``accept`` value.