[docs] Mismatched terminology in Oauth flow

[jhuleatt]

In install flow, it says "API Key":

Firebase/extension.yaml

Lines 162 to 163 in 623b359

	What is your Mailchimp API key? To obtain a Mailchimp API key, go to your
	[Mailchimp account](https://storage.googleapis.com/firebase-mailchimp/index.html).

In result of Oauth flow, it says "Access token"

Which is the correct term that both should use?

[barticus]

Closing as resolved in linked PR.