Update README.md

README.md

@@ -81,7 +81,7 @@ The expert written references were published by the U.S. National Gallery of Art
 
 #### Annotation process
 
-The expert written references descriptions were composed according to the U.S. National Gallery of Art's Accessibility Guidelines: https://www.nga.gov/visit/accessibility/collection-image-descriptions.
+The expert written reference descriptions were composed according to the U.S. National Gallery of Art's Accessibility Guidelines: https://www.nga.gov/visit/accessibility/collection-image-descriptions.
 
 #### Who are the annotators?