[#4204] Fixing slight markup issue

Author: weaverryan

reference/constraints/Expression.rst

@@ -221,7 +221,7 @@ more about the expression language syntax, see
         In Symfony 2.6, the Expression constraint *is* executed if the value
         is ``null``. Before 2.6, if the value was ``null``, the expression
         was never executed and the value was considered valid (unless you
-        also had a constraint like `NotBlank` on the property).
+        also had a constraint like ``NotBlank`` on the property).
 
 For more information about the expression and what variables are available
 to you, see the :ref:`expression <reference-constraint-expression-option>`