SubmittingPatches: clarify GitHub artifact format

GitHub wraps artifacts generated by workflows in a .zip file.

Internally, workflows can package anything they like in them.

A recently generated failure artifact had the form:

windows-artifacts.zip
  Length      Date    Time    Name
---------  ---------- -----   ----
 76001695  12-19-2023 01:35   artifacts.tar.gz
 11005650  12-19-2023 01:35   tracked.tar.gz
---------                     -------
 87007345                     2 files

Signed-off-by: Josh Soref <jsoref@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: jsoref

Documentation/SubmittingPatches

@@ -606,7 +606,8 @@ If a branch does not pass all test cases then it will be marked with a
 red +x+, instead of a green check. In that case, you can click on the
 failing job and navigate to "ci/run-build-and-tests.sh" and/or
 "ci/print-test-failures.sh". You can also download "Artifacts" which
-are tarred (or zipped) archives with test data relevant for debugging.
+are zip archives containing tarred (or zipped) archives with test data
+relevant for debugging.
 
 Then fix the problem and push your fix to your GitHub fork. This will
 trigger a new CI build to ensure all tests pass.