Fix typo with v3 artifact downloads in migration guide

[robherley]

In the migration guide, the download for the v3 was as follows:

      - name: Download All Artifacts
        uses: actions/download-artifact@v3
        with:
          path: my-artifact
      - run: ls -R my-artifact

And it was stated that it would create the following directory:

my-artifact/
  file-macos-latest.txt
  file-ubuntu-latest.txt
  file-windows-latest.txt

This was incorrect, since no name was specified, it actually made an inner directory due to the behavior outlined here in the v3 docs.

If the name input parameter is not provided, all artifacts will be downloaded. To differentiate between downloaded artifacts, a directory denoted by the artifacts name will be created for each individual artifact.

Instead we must provide both the name and the path to have it match the scenario we outlined.

More context: #478 (comment)

[jsoref]

Migrating says "The new merge-multiple: input will support downloading multiple artifacts within the same directory." That isn't technically wrong, but from the perspective of migrating users, it is.

It would be better to say:
"If you previously used name:, you will need to replace it with merge-multiple: true -- noting that these are not precisely equivalent."