[engsys] docs,fix: --root is required for tox run -e black

[kdestin]

Description

This pull request updates eng_sys_checks.md to specify that the --root parameter needs to point to a package's root directory when running tox run -e black.

cc: @scbedd

Background

tox run -e black used to be a thin wrapper around an invocation of black, which meant we didn't need to specify --root so long as positional arguments could be forwarded to the underlying invocation of black.

#38212 updated tox run -e black to instead run a script which invokes black. The script depends on the dev-requirements.txt in a package's root directory, which makes it mandatory to specify --root now.

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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[scbedd]

You're absolutely right thanks for the catch @kdestin