[UI] Fix bad command line appearance

The command displayed as grey on grey and was not readable

Author: kevin-verschaeve

Diff for: core/openapi.md

@@ -64,7 +64,7 @@ class Book {}
 
 Then, either use the query parameter for the web version such as `/docs?filter_tags[]=customer` or through the command line:
 
-```
+```console
 bin/console api:openapi:export --filter-tags=customer
 ```