Update documentation to use tox 4 command line syntax

See: https://tox.wiki/en/latest/upgrading.html#updating-usage-with-e

mentioned in issue swh/meta#4959 (closed)

Jenkins job DDOC/builds #120 succeeded .
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And works on the Tox 3 version on Bullseye; so that's fine with me

approved this merge request

added 2 commits

• 26e47563 - 1 commit from branch swh/devel:master
• e9687939 - Update documentation to use tox 4 command line syntax

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enabled an automatic merge when the pipeline for e9687939 succeeds

Jenkins job DDOC/builds #131 failed .
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aborted the automatic merge because target branch was updated

added 3 commits

• e9687939...0b1d5834 - 2 commits from branch swh/devel:master
• 2ef3ac07 - Update documentation to use tox 4 command line syntax

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enabled an automatic merge when the pipeline for 2ef3ac07 succeeds

Jenkins job DDOC/builds #139 succeeded .
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merged