docs: improve description

Author: kenjis

user_guide_src/source/libraries/validation.rst

@@ -609,7 +609,7 @@ a boolean true or false value signifying true if it passed the test or false if
 .. literalinclude:: validation/034.php
 
 By default, the system will look within ``CodeIgniter\Language\en\Validation.php`` for the language strings used
-within errors. In custom rules, you may provide error messages by accepting a ``$error`` variable by reference in the
+within errors. In custom rules, you may provide error messages by accepting a ``&$error`` variable by reference in the
 second parameter:
 
 .. literalinclude:: validation/035.php
@@ -630,7 +630,7 @@ for rules like ``required_with`` that needs to check the value of another submit
 
 .. literalinclude:: validation/037.php
 
-Custom errors can be returned as the fourth parameter, just as described above.
+Custom errors can be returned as the fourth parameter ``&$error``, just as described above.
 
 .. _validation-using-closure-rule: