Fix for the Exploration timeline playheads (#982)

**Related Ticket:**
[981](#981)

### Description of Changes
Fix the `selectedCompareDate` not being sent in the API requests,
resulting in a the comparison being broken.

### Notes & Questions About Changes
_{Add additonal notes and outstanding questions here related to changes
in this pull request}_

### Validation / Testing
_{Update with info on what can be manually validated in the Deploy
Preview link for example "Validate style updates to selection modal do
NOT affect cards"}_

app/scripts/components/exploration/components/map/index.tsx

@@ -173,7 +173,7 @@ export function ExplorationMap(props: ExplorationMapProps) {
           {selectedDay && (
             <ExplorationMapLayers
               datasets={loadedDatasets}
-              selectedDay={selectedDay}
+              selectedDay={selectedCompareDay}
               idSuffix='-compare'
             />
           )}