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.

Update authentication/oauth-mattermost.md, which currently documents the callback URL as /auth/oauth2/callback. Use the redirect_uri shown in the issue as the reference, change the documented Mattermost callback URL to /auth/mattermost/callback, and verify that the documentation matches the working setup described.

由索引模型根据 Issue 内容生成。