[e.cash] Remove mention of bits for the denomination

Summary: The term "bits" is not widely used in the eCash community.

Test Plan:
Read the pages to make sure they read well
git grep bits

Reviewers: #bitcoin_abc, bytesofman

Reviewed By: #bitcoin_abc, bytesofman

Subscribers: bytesofman

Differential Revision: https://reviews.bitcoinabc.org/D14694