bump up title.font.size in range_slider_top_axis

... to make margin "push" on all machine. Previously the dflt
    margin.t of 100 was enough to fit the xaxis.title on etpinard's
    laptop and presumably on CI as well.

Author: etpinard

test/image/baselines/range_slider_top_axis.png

@@ -23,7 +23,10 @@
   ],
   "layout": {
     "xaxis": {
-      "title": "Top X Axis",
+      "title": {
+        "text": "Top X Axis",
+        "font": {"size": 24}
+      },
       "rangeslider": {
         "visible": true
       },