[VisionTextDualEncoder] Fix doc example
# What does this PR do?

In `modeling_vision_text_dual_encoder.py`, this line (doc example) will fail
```
>>> loss, logits_per_image = outputs.loss, outputs.logits_per_imag
```

It should be `outputs.logits_per_image` (`CLIPOutput`)

## Who can review?

@patil-suraj 