monitoring: Fix example writeRelabelConfig field name in remoteWrite documentation

The field is called `sourceLabels` not `source_labels`.

Signed-off-by: Jan Fajerski <[email protected]>

Author: jan--f

modules/monitoring-configuring-remote-write.adoc

@@ -170,7 +170,7 @@ data:
       remoteWrite:
       - url: "https://remote-write.endpoint"
         writeRelabelConfigs:
-        - source_labels: [__name__]
+        - sourceLabels: [__name__]
           regex: 'my_metric'
           action: keep