README: Fix link to API docs (#295)

Follow-up to #294.

Author: olleolleolle

README.md

@@ -30,7 +30,7 @@ end
 
 ## Documentation
 
-- [API](https://ruby-doc.org/stdlib/libdoc/csv/rdoc/CSV.html):  all classes, methods, and constants.
+- [API](https://ruby.github.io/csv/):  all classes, methods, and constants.
 - [Recipes](https://ruby.github.io/csv/doc/csv/recipes/recipes_rdoc.html):  specific code for specific tasks.
 
 ## Development