Fixed a syntax issue

Author: javiereguiluz

form/unit_testing.rst

@@ -7,7 +7,7 @@ How to Unit Test your Forms
 .. caution::
 
     This article is intended for developers who create
-    :doc:`custom form types </form/create_custom_field_type>`_. If you are using
+    :doc:`custom form types </form/create_custom_field_type>`. If you are using
     the :doc:`built-in Symfony form types </reference/forms/types>` or the form
     types provided by third-party bundles, you don't need to unit test them.