update image for Dashboards Alerting plugin (opensearch-project#1791)

* new image of Dashboards

Signed-off-by: alicejw <[email protected]>

* update link to new image

Signed-off-by: alicejw <[email protected]>

* fix figure title

Signed-off-by: alicejw <[email protected]>

Signed-off-by: alicejw <[email protected]>

Author: alicejw1

_monitoring-plugins/alerting/index.md

@@ -19,4 +19,6 @@ The monitor you create notifies you when data from one or more OpenSearch indexe
 
 To get started, choose **Alerting** in OpenSearch Dashboards.
 
-![OpenSearch Dashboards side bar with link]({{site.url}}{{site.baseurl}}/images/alerting.png)
+![OpenSearch Dashboards side bar with link]({{site.url}}{{site.baseurl}}/images/dashboards-nav.png)
+
+***Figure 1: Alerting plugin in OpenSearch Dashboards***