Fix Mattermost OAuth2 documentation

[Chostakovitch]

The documentation found here states that the callback URL should be <your-codimd-url>/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://<mattermost url>/oauth/authorize?response_type=code&redirect_uri=<codimd url>/auth/mattermost/callback&client_id=<client id>

So that the callback URL should be <your-codimd-url>/auth/mattermost/callback. It works with this setup.