fix Doxygen intro

Summary:
I noticed that the intro at doc.bitcoinabc.org is outdated
This fixes it.

Test Plan: Read it.

Reviewers: schancel, jasonbcox, deadalnix, #bitcoin_abc

Reviewed By: jasonbcox, #bitcoin_abc

Subscribers: teamcity

Differential Revision: https://reviews.bitcoinabc.org/D1417