Fix remaining Java API reference link

Author: dacharyc

source/includes/api-details/java/atlas/call-atlas-function-call-function-description.rst

@@ -1,7 +1,7 @@
 To execute a function from the SDK, use the
 :java-sdk:`getFunctions() <io/realm/mongodb/App.html#getFunctions(io.realm.mongodb.User)>`
 method of the your :java-sdk:`App <io/realm/mongodb/App.html>`
-to retrieve a :java-sdk:`Functions manager <>`.
+to retrieve a :java-sdk:`Functions manager <io/realm/mongodb/functions/Functions.html>`.
 
 Pass the name and parameters of the function you would like to call to
 :java-sdk:`callFunction() <io/realm/mongodb/functions/Functions.html#callFunction(java.lang.String,java.util.List,java.lang.Class)>`