Clarify Subscription page language

Summary: This says "Edit Subscription" but the page only lets you update the payment method for autopay. I think this is confusing users. I tried to find the best language here, but suggest something else if you prefer.

Test Plan: Review language on sample subscription page.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Maniphest Tasks: T12451

Differential Revision: https://secure.phabricator.com/D17944