docs: fix RST format

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -412,7 +412,7 @@ Or as a labeled style:
 .. literalinclude:: validation/024.php
 
 If you'd like to include a field's "human" name, or the optional parameter some rules allow for (such as max_length),
-or the value that was validated you can add the ``{field}``, ``{param}`` and ``{value}`` tags to your message, respectively:
+or the value that was validated you can add the ``{field}``, ``{param}`` and ``{value}`` tags to your message, respectively::
 
     'min_length' => 'Supplied value ({value}) for {field} must have at least {param} characters.'