[WEBSITE] Update website description from Bitcoin Cash to eCash

Summary: This appears in the title bar and google search result and has been missed so far.

Test Plan:

make serve

Check the title bar displays eCash.

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: Klakurka

Differential Revision: https://reviews.bitcoinabc.org/D9866