Fix missing comma in docstring example (plotly#1583)

Author: Jo-Con-El

Diff for: plotly/offline/offline.py

@@ -390,7 +390,7 @@ def plot(figure_or_data, show_link=False, link_text='Export to plot.ly',
     plot([go.Scatter(x=[1, 2, 3], y=[3, 2, 6])], filename='my-graph.html')
     # We can also download an image of the plot by setting the image parameter
     # to the image format we want
-    plot([go.Scatter(x=[1, 2, 3], y=[3, 2, 6])], filename='my-graph.html'
+    plot([go.Scatter(x=[1, 2, 3], y=[3, 2, 6])], filename='my-graph.html',
          image='jpeg')
     ```
     More examples below.