[1.x][Bug] fix timeline icon in Visualize list

Icon was not loading in the Visualize list because it was
referencing a non-existant icon. Other parts of the code
loads the icon correctly, so updated the icon to an existing
icon.

Issues resolved:
#658

Backport PR:
#659

Signed-off-by: Kawika Avilla <kavilla414@gmail.com>

src/plugins/vis_type_timeline/public/timeline_vis_type.tsx

@@ -53,7 +53,7 @@ export function getTimelineVisDefinition(dependencies: TimelineVisDependencies)
   return {
     name: TIMELINE_VIS_NAME,
     title: 'Timeline',
-    icon: 'visTimeline',
+    icon: 'visTimelion',
     description: i18n.translate('timeline.timelineDescription', {
       defaultMessage: 'Build time-series using functional expressions',
     }),