Change params._source to params['_source'] in example.
@@ -38,7 +38,7 @@ the above case), and allow to return custom values to be returned (the
evaluated value of the script).
Script fields can also access the actual `_source` document and
-extract specific elements to be returned from it by using `params._source`.
+extract specific elements to be returned from it by using `params['_source']`.
Here is an example:
[source,js]
