update API link (#2812)

Author: jordan-smith721

docs/index.txt

@@ -61,7 +61,7 @@ For tutorials on Mongoid, see the `Mongoid Manual <https://mongodb.com/docs/mong
     reference/connection-and-configuration
     reference/working-with-data
     reference/schema-operations
-    API <https://mongodb.com/docs/ruby-driver/master/api/>
+    API <https://mongodb.com/docs/ruby-driver/current/api/>
     release-notes
     reference/additional-resources
     contribute