[website] Add missing div tag on ecash page

Summary:
Seems it was left out accidentally.
It made the justification in the bottom few sections a bit wonky.

Test Plan: Ran it locally, looked at it. Looks better.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Differential Revision: https://reviews.bitcoinabc.org/D9826