[DOC] clang-*-8 tools are now in the backport repository for Debian 10

Summary:
I figured this out recently while setting up a new Debian box. These
packages are no longer part of the default sources, so the
CONTRIBUTING.md file needs to be updated accordingly.

Test Plan:
Read the doc and follow the instructions.
Tested the rendering with grip CONTRIBUTING.md.

Reviewers: #bitcoin_abc, majcosta, jasonbcox

Reviewed By: #bitcoin_abc, majcosta, jasonbcox

Differential Revision: https://reviews.bitcoinabc.org/D7074