Fix output file path - remove subdirectory prefix

Changed SaveImage filename_prefix from 'Zimage_Img2Img/ComfyUI_' to 'output'
This fixes FileNotFoundError when retrieving the processed image.

simple_api_workflow.json

@@ -380,7 +380,7 @@
         "581",
         0
       ],
-      "filename_prefix": "Zimage_Img2Img/ComfyUI_"
+      "filename_prefix": "output"
     }
   }
 }