change deprecated message

Co-authored-by: Dai MIKURUBE <dmikurube@users.noreply.github.com>

Author: giwa

src/main/java/org/embulk/output/s3/S3FileOutputPlugin.java

@@ -194,7 +194,7 @@ private ClientConfiguration getClientConfiguration(PluginTask task)
             // set http proxy
             // backward compatibility
             if (task.getProxyHost().isPresent()) {
-                logger.warn("proxy_host is deprecated. use http_proxy.host instead");
+                logger.warn("Configuration with \"proxy_host\" is deprecated. Use \"http_proxy.host\" instead.");
                 if (!task.getHttpProxy().isPresent()) {
                     ConfigMapper configMapper = CONFIG_MAPPER_FACTORY.createConfigMapper();
                     ConfigSource configSource = CONFIG_MAPPER_FACTORY.newConfigSource();