docs: Remove trailing '/' from Authentik issuer example (#4646)

The Authentik provider already adds a `/` after the `issuer`, so this creates a double slash that causes a NextAuth `SIGNIN_OAUTH_ERROR` ("expected 200 OK, got: 301 Moved Permanently").

docs/docs/providers/authentik.md

@@ -31,5 +31,5 @@ providers: [
 ```
 
 :::note
-`issuer` should include the slug – e.g. `https://my-authentik-domain.com/application/o/My_Slug/`
+`issuer` should include the slug without a trailing slash – e.g., `https://my-authentik-domain.com/application/o/My_Slug`
 :::