Update README.md

Fixed the README to reflect the actual configuration.

README.md

@@ -153,7 +153,7 @@ All of annotations use English as primary language.
 When loading a specific configuration, users has to append a version dependent suffix:
 ```python
 from datasets import load_dataset
-load_dataset("visual_genome", "region_description_v1.2.0")
+load_dataset("visual_genome", "region_descriptions_v1.2.0")
 ```
 
 #### region_descriptions