Fix example in README

https://github.com/avh4/elm-github-v3/blob/f15805930c4498345560984249d916dff0ad09ac/README.md?plain=1#L20

fixed
- the error in type and
- usage of `Github.oauthToken`

```elm
import Github

getPullRequestTitles : Cmd (Result Http.Error (List String))
getPullRequestTitles =
Github.getPullRequests
{ authToken = Github.oauthToken "123..."
, repo = "avh4/elm-format"
}
|> Task.map (List.map .title)
|> Task.attempt identity
```

Open README.md at the linked line and compare the example with the reported corrections. Update the example so its type and Github.oauthToken usage match the shown Elm snippet, then verify the README displays the intended working usage.

Written by the indexing model from the issue text.