Update server API

Added new URL to the server api due to changes in the server set up

app/src/main/java/project/schafkopfzaehler_2/api_server.java

 public interface api_server{
 
         @Multipart
-        @POST("/face_detection/detect/")
+        @POST("/Kartenerkennung/detect/")
 
         Call<POJOClass> uploadAttachment(@Part MultipartBody.Part filePart);