Update example

When I was reading through the example code, fixed 3 things:

* Fixed missing semicolon.
* A lowercase doctype is perfectly fine, and is more consistent with the other html code.
* A double charset declaration was included (the "old" way, and in https://github.com/t4t5/sweetalert/commit/f5cc61bd0bdf5b84889ae6e880ad1a3c53cfd15f @simonVansuyt included the "new" way).