docs: In-App Billing HTML formatting cleanup

Fixed line lengths and other white-space-related fixes (e.g.
removing trailing lines and spaces). Also fixed a couple of typos.

I'll build and stage it (see first comment for stage location) but
if it all checks out I'll go ahead and +2 it and submit it myself.

Change-Id: Ia47dfa3b69c87aed01445551ba3259e9f4ba3c44