Fix examples in VisualBERT docs
This PR fixes the examples in [VisualBERT docs](https://huggingface.co/transformers/master/model_doc/visual_bert.html).
The issue is described in this [comment](https://github.com/huggingface/transformers/pull/10534#issuecomment-853015418) by @NielsRogge.

Requesting review from @sgugger