update R API documentation for `mlflow_register_external_observer()` (#3549)
update R API documentation for `mlflow_register_external_observer()` (#3549)

Signed-off-by: Yitao Li <506e136862ca6a2cd6209c2ab7376874a235e7c2@rstudio.com>
mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow,mlflow/mlflow