Change early stopped marker to "x" in ProgressionPlot (#3384)

Summary:
Pull Request resolved: #3384

As titled. Looks a lot better.

Reviewed By: saitcakmak

Differential Revision: D69801380

fbshipit-source-id: 993be3739313d065fb71ab2b5d7bb2eee3fb9fb1

ax/analysis/plotly/progression.py

@@ -121,8 +121,8 @@ def compute(
                     x=terminal_points[x_axis_name],
                     y=terminal_points[metric_name],
                     mode="markers",
+                    marker={"symbol": "x", "color": "red"},
                     showlegend=False,
-                    line_color="red",
                     hoverinfo="none",
                 )
             )