[release/v1.3] docs: fixed kafka config example (#2370)

Example shows `loki.source.kafka "local"` pointing to `loki.relabel.kafka.receiver`. This leads to no new label being added. Correct example should have the kafka source pointing directly to `loki.write.local.receiver`

(cherry picked from commit 672dfe7)

Co-authored-by: Jay Clifford <[email protected]>

docs/sources/reference/components/loki/loki.source.kafka.md

@@ -145,7 +145,7 @@ loki.source.kafka "local" {
   brokers                = ["localhost:9092"]
   topics                 = ["quickstart-events"]
   labels                 = {component = "loki.source.kafka"}
-  forward_to             = [loki.relabel.kafka.receiver]
+  forward_to             = [loki.write.local.receiver]
   use_incoming_timestamp = true
   relabel_rules          = loki.relabel.kafka.rules
 }