Fix Explode validator not appearing on "Standard Validation Classes" list

[renanliberato]

I've opened an already merged pull request with the Explode validator documentation.
However, now that it's on the website I've seen that I forgot to include it in the "Standard Validation Classes". I'm really sorry.

Before correcting it, I'd like to ask if I should open a pull request just to this issue or would it be more worth to include the correction in a pull request with something more, like #172 - DateStep documentation.

Thank you very much for your attention.

[froschdesign]

Fixed with #248