Fix Checkstyle Errors :(

Co-authored-by: Christoph <tobi@urpost.de>

src/main/java/mops/gruppen2/domain/User.java

@@ -9,7 +9,7 @@ import lombok.Getter;
 @EqualsAndHashCode(exclude = {"givenname", "familyname", "email"})
 public class User {
 
-    private final String user_id;
+    private final String id;
     private final String givenname;
     private final String familyname;
     private final String email;