Fix bookmark to authentication in docs

Author: bradymholt

README.md

@@ -29,7 +29,7 @@ import ynab
 ## Usage
 
 To use this client, you must
-[obtain an access token](https://api.ynab.com/#authentication-overview) from
+[obtain an access token](https://api.ynab.com/#authentication) from
 the [Account Settings](https://app.ynab.com/settings) area of the YNAB web
 application.