Fix AttributeError in train_dreambooth_lora_flux2_img2img validation

The validation block referenced `args.validation_image_path`, but the CLI
argument is defined as `--validation_image`, so argparse stores it as
`args.validation_image`. When a user passes `--validation_prompt`, the
script crashes immediately with `AttributeError: 'Namespace' object has
no attribute 'validation_image_path'` before any training starts.

Other Flux2 img2img-style dreambooth scripts (e.g.
`train_dreambooth_lora_flux2_klein_img2img.py` and
`train_dreambooth_lora_flux_kontext.py`) already use the correct
`args.validation_image`; this just aligns this script with them.

Author: Ricardo-M-L

examples/dreambooth/train_dreambooth_lora_flux2_img2img.py

@@ -1483,7 +1483,7 @@ def _encode_single(prompt: str):
                 )
 
     if args.validation_prompt is not None:
-        validation_image = load_image(args.validation_image_path).convert("RGB")
+        validation_image = load_image(args.validation_image).convert("RGB")
         validation_kwargs = {"image": validation_image}
         if args.remote_text_encoder:
             validation_kwargs["prompt_embeds"] = compute_remote_text_embeddings(args.validation_prompt)