update NextCloud oauth link

In my Nextcloud 18 it only works with `https://cloud.example.com/apps/sociallogin/custom_oauth2/Mailcow` instead of `https://cloud.example.com/index.php/apps/sociallogin/custom_oauth2/Mailcow`.

docs/third_party-nextcloud.md

@@ -4,7 +4,7 @@ In the following, we will only assume that you have already set up NextCloud at
 To set up authentication via mailcow, you can use OAuth2 as described below. 
 
 1. Log into mailcow as administrator.
-2. Scroll down to _OAuth2 Apps_ and click the _Add_ button. Specify the redirect URI as `https://cloud.example.com/index.php/apps/sociallogin/custom_oauth2/Mailcow` and click _Add_. Save the client ID and secret for later.
+2. Scroll down to _OAuth2 Apps_ and click the _Add_ button. Specify the redirect URI as `https://cloud.example.com/index.php/apps/sociallogin/custom_oauth2/Mailcow` (for NextCloud 18 without `index.php`) and click _Add_. Save the client ID and secret for later.
 3. Log into NextCloud as administrator.
 4. Click the button in the top right corner and select _Apps_. Click the search button in the toolbar, search for the [_Social Login_](https://apps.nextcloud.com/apps/sociallogin) plugin and click _Download and enable_ next to it.
 5. Click the button in the top right corner and select _Settings_. Scroll down to the _Administration_ section on the left and click _Social login_.