update config attributes' names for including linkage

Author: Azdaroth

README.md

@@ -1281,8 +1281,8 @@ JSONAPI.configure do |config|
 
   # Resource Linkage
   # Controls the serialization of resource linkage for non compound documents
-  # NOTE: always_include_has_many_linkage_data is not currently implemented
-  config.always_include_has_one_linkage_data = false  
+  # NOTE: always_include_to_many_linkage_data is not currently implemented
+  config.always_include_to_one_linkage_data = false
 end
 ```