enclose data type with double backticks

Author: xabbuh

reference/forms/types/options/data.rst.inc

@@ -1,7 +1,7 @@
 data
 ~~~~
 
-**type**: mixed **default**: Defaults to field of the underlying object (if there is one)
+**type**: ``mixed`` **default**: Defaults to field of the underlying object (if there is one)
 
 When you create a form, each field initially displays the value of the
 corresponding property of the form's domain object (if an object is bound