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

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