Simplify install instructions for linter dependencies

Summary:
This patch makes this section of the doc more consistent
and less repetitive. It also reduces the number of copy-paste
install steps from two to one on recent Ubuntu and Debian distros.

Test Plan: Read it.

Reviewers: #bitcoin_abc, Fabien

Reviewed By: #bitcoin_abc, Fabien

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D6389