Update external-kafka.mdx

Co-authored-by: Burak Yigit Kaya <[email protected]>

Author: aldy505

develop-docs/self-hosted/experimental/external-kafka.mdx

@@ -110,6 +110,6 @@ vroom:
     SENTRY_KAFKA_SSL_CERT_PATH: ""
     SENTRY_KAFKA_SSL_KEY_PATH: ""
     SENTRY_KAFKA_SASL_MECHANISM: "PLAIN" # Valid options are PLAIN, SCRAM-SHA-256, SCRAM-SHA-512.
-    SENTRY_KAFKA_SASL_USERNAME: "username"
-    SENTRY_KAFKA_SASL_PASSWORD: "password"
+    SENTRY_KAFKA_SASL_USERNAME: "<username>"
+    SENTRY_KAFKA_SASL_PASSWORD: "<password>"
 ```