Update data-validation.rst

From the source:

https://github.com/cakephp/cakephp/blob/master/lib/Cake/Utility/Validation.php#L731

return ($check > $lower && $check < $upper);

It's wrong in the 1.3 docs too, which I cannot PR on, if the source if not entirely different (have'nt checked)
Merge pull request #1918 from hmic/patch-3

Update data-validation.rst