fix: use lowercase config key in Javadoc example

Fixes Copilot review comment: ConfigInitializer maps fields to
lowercase keys, so the example should read
plugin.httpclient.propagation_exclude_ports instead of uppercase.

Author: daguimu

apm-sniffer/apm-sdk-plugin/httpclient-commons/src/main/java/org/apache/skywalking/apm/plugin/httpclient/HttpClientPluginConfig.java

@@ -43,7 +43,7 @@ public static class HttpClient {
              * <p>Default: {@code "8123"} (ClickHouse HTTP interface).
              *
              * <p>Example – also exclude port 9200 (Elasticsearch):
-             * {@code plugin.httpclient.PROPAGATION_EXCLUDE_PORTS=8123,9200}
+             * {@code plugin.httpclient.propagation_exclude_ports=8123,9200}
              */
             public static String PROPAGATION_EXCLUDE_PORTS = "8123";
         }