Fix various typos or naming inconsistencies

Regarding "Payment Request"/"payment request"/"PaymentRequest", I noticed we were not using a consistent style, so here's an attempt to improve this by using lowercase "payment request" when referring to the whole payment request process , or "PaymentRequest" when directly referring to the first API Call.

As always, please let me know if you think anything I suggest is wrong :) .

(I can update the Style Guide if we agree on the "PaymentRequest" writing rule)