This will fix a dead link on https://www.bitcoinabc.org/electrum/CONTRIBUTING.html and use the same doc format everywhere.

I'm not sure I agree about everything in this document, but let's not mix format changes with documentation updates. The only changes in the document this diff introduces are:

• a couple of Electron Cash --> Electrum ABC fixes (undetected previously because they were split over two lines)
• remove the note about the oldest version that supports external plugins (this was before the BCH split, so no such Electrum ABC version exists))

Check rendering in my IDE, test that the links are not dead