rename config param for multiple vision encoders

docs/source/multitask_dit.mdx

@@ -157,7 +157,7 @@ The model supports two positional encoding methods for action sequences:
 # Use separate vision encoder per camera
 # This may be useful when cameras have significantly different characteristics, but
 # be wary of increased VRAM footprint.
---policy.use_separate_encoder_per_camera=true
+--policy.use_separate_rgb_encoder_per_camera=true
 
 # Image preprocessing
 --policy.image_resize_shape=[XXX,YYY] \ # you may need to resize your images for inference speed ups