use snake_case instead of under_score

Author: wing328

modules/swagger-codegen/src/main/java/io/swagger/codegen/languages/PythonClientCodegen.java

@@ -61,7 +61,7 @@ public PythonClientCodegen() {
                         "return", "def", "for", "lambda", "try"));
 
         cliOptions.clear();
-        cliOptions.add(new CliOption("packageName", "python package name (convention: under_score), default: swagger_client"));
+        cliOptions.add(new CliOption("packageName", "python package name (convention: snake_case), default: swagger_client"));
         cliOptions.add(new CliOption("packageVersion", "python package version, default: 1.0.0"));
     }