examples: use ExampleAlert instead of Watchdog

Replace Watchdog alerts part of the `example-group` in some examples by
ExampleAlert alerts to reinforce the fact that this is just an example.

Signed-off-by: Damien Grisonnet <dgrisonn@redhat.com>

examples/existingrule.yaml

@@ -1,9 +1,9 @@
 groups:
 - name: example-group
   rules:
-  - alert: Watchdog
+  - alert: ExampleAlert
     expr: vector(1)
     labels:
-      severity: "none"
+      severity: "warning"
     annotations:
-      description: This is a Watchdog meant to ensure that the entire alerting pipeline is functional.
+      description: This is an example alert.