[Docs] Add language identifiers to fenced code blocks (#28955)

Add language identifiers to code blocks

examples/research_projects/bertabs/README.md

@@ -8,7 +8,7 @@ The model is loaded with the pre-trained weights for the abstractive summarizati
 
 ## Setup
 
-```
+```bash
 git clone https://github.com/huggingface/transformers && cd transformers
 pip install .
 pip install nltk py-rouge