Fix Mattermost OAuth2 documentation

The documentation found [here](https://github.com/hackmdio/codimd-docs/blob/master/authentication/oauth-mattermost.md) states that the callback URL should be `/auth/oauth2/callback`.

When using this configuration in Mattermost, the following error is raised : `invalid_request: Supplied redirect_uri did not match registered callback_url`.

Indeed, when inspecting the request to Mattermost, the URL looks like :

```
https:///oauth/authorize?response_type=code&redirect_uri=/auth/mattermost/callback&client_id=
```

So that the callback URL should be `/auth/mattermost/callback`. It works with this setup.