wrap bibtex citation in code block

README.md

@@ -228,6 +228,7 @@ See published code on [GithHub](https://github.com/qurator-spk/sbb_ner).
 
 **BibTeX:**
 
+```bibtex
 @article{labusch_bert_2019,  
 	title = {{BERT} for {Named} {Entity} {Recognition} in {Contemporary} and {Historical} {German}},  
 	volume = {Conference on Natural Language Processing},  
@@ -238,6 +239,7 @@ See published code on [GithHub](https://github.com/qurator-spk/sbb_ner).
 	year = {2019},  
 	pages = {9},  
 }
+```
 
 **APA:**