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.json

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