exp list: fix git_remote metavar

[dberenbaum]

Before:

$ dvc exp list --help
usage: dvc experiments list [-h] [-q | -v] [-A] [--rev <commit>] [-n <num>]
                            [--name-only]
                            [[<git_remote>]]

List local and remote experiments.
Documentation: <https://man.dvc.org/exp/list>

positional arguments:
  [<git_remote>]        Optional Git remote name or Git URL. If provided, experiments
                        from the specified Git repository will be listed instead of
                        local ones.

After:

$ dvc exp list --help
usage: dvc experiments list [-h] [-q | -v] [-A] [--rev <commit>] [-n <num>]
                            [--name-only]
                            [<git_remote>]

List local and remote experiments.
Documentation: <https://man.dvc.org/exp/list>

positional arguments:
  <git_remote>          Optional Git remote name or Git URL. If provided, experiments
                        from the specified Git repository will be listed instead of
                        local ones.