Fix documentation for stripe: Gateway "stripe_checkout" and "stripe_js" does not exist

[KRens]

To fix:
https://github.com/Payum/Payum/blob/master/docs/stripe/checkout.md
https://github.com/Payum/Payum/blob/master/docs/stripe/js.md

What to fix:
Replace:
->addGateway('gatewayName', [
'factory' => 'stripe_checkout',

    with:
        ->addGateway('stripe_checkout', [
    'factory' => 'stripe_checkout',

I just spend 3 hours trying to figure out why it did not work.
The prepare was refarring to stripe_checkout, as documented, but the gatewayname needed to be changed. This was not mentioned in the documentation!

[pierredup]

@KRens Can you please open a PR with the changes? Thanks.

[KRens]

Sorry, I'm more a manager than a developer. I don't even have git here. It will be faster for you to change probably.