DOC: fix typo in user guide
@@ -69,7 +69,7 @@ JSON format
The fundamental chart representation output by Altair is a JSON string format;
one of the core methods provided by Altair is :meth:`Chart.to_json`, which
returns a JSON string that represents the chart content.
-Additinoally, you can save a chart to a JSON file using :meth:`Chart.save`,
+Additionally, you can save a chart to a JSON file using :meth:`Chart.save`,
by passing a filename with a ``.json`` extension.
For example, here we save a simple scatter-plot to JSON:
