[doc] Update Doxygen to mention eCash instead of Bitcoin Cash

Summary: https://www.bitcoinabc.org/doc/dev/

Test Plan:
Read the file
Did git grep Cash to check for other instances.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D11197