Update external-kafka.mdx

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

Author: aldy505

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

@@ -55,8 +55,8 @@ x-snuba-defaults: &snuba_defaults
     KAFKA_SSL_CERT_PATH:
     KAFKA_SSL_KEY_PATH:
     KAFKA_SASL_MECHANISM: "PLAIN" # Valid options are PLAIN, SCRAM-SHA-256, SCRAM-SHA-512.
-    KAFKA_SASL_USERNAME: "username"
-    KAFKA_SASL_PASSWORD: "password"
+    KAFKA_SASL_USERNAME: "<username>"
+    KAFKA_SASL_PASSWORD: "<password>"
 ```
 
 If you encounter any failing startup, try to use `consumer` instead of `rust-consumer`.