fix ReST syntax.

doc/library/config.txt

@@ -474,6 +474,7 @@ import theano and print the config variable, as in:
 
     When not ``'off'``, the value of this option dictates what happens when
     an Op's inputs do not provide appropriate test values:
+
         - ``'ignore'`` will silently skip the debug mechanism for this Op
         - ``'warn'`` will raise a UserWarning and skip the debug mechanism for
           this Op