Rename Paystack key to public_key

[iKey]

First of all, great work mate.

Just a tiny correction, the paystack key required for inline initialization is the public key. Kindly rename the paystack key variable to public_key or at least, just add a comment somewhere to notify users that it's the public key they need to use there.

Regards,

[Olusegun Ayeni]

@eyeke04 thanks a lot. I will put a comment in readme.