The help message of the 'git-pr' sub-command misses the sub-command name

When we use the 'git-pr' sub-command, such as `git-pr list -h`, the output information missed the command name `list`.

```
$ git-pr list -h

usage: git-pr [options] [<ID>] // <---- Here miss the `list`
      // Ignore the following output.
```

The first line `usage: git-pr [options] [<ID>]` should be `usage: git-pr list [options] [<ID>]`.

The wrong sub-command is listed below:

git-pr apply
git-pr approve // will change to git-pr review
git-pr checkout
git-pr close
git-pr create
git-pr fetch
git-pr help
git-pr info
git-pr integrate
git-pr issue
git-pr list
git-pr set
git-pr show
git-pr sponsor