docs/system: Fix the information on how to run certain functional tests

docs/system: Fix the information on how to run certain functional tests

The tests have been converted to the functional framework, so
we should not talk about Avocado here anymore.

Fixes: f7d6b772200 ("tests/functional: Convert BananaPi tests to the functional framework")
Fixes: 380f7268b7b ("tests/functional: Convert the OrangePi tests to the functional framework")
Fixes: 4c0a2df81c9 ("tests/functional: Convert some tests that download files via fetch_asset()")
Message-ID: <20250311160847.388670-1-thuth@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>