Fixed: typos about containers in linear foreast notebook

Author: djarpin

introduction_to_applying_machine_learning/linear_time_series_forecast/linear_time_series_forecast.ipynb

@@ -315,7 +315,7 @@
     "---\n",
     "## Train\n",
     "\n",
-    "Now we can begin to specify our linear model.  First, let's specify the containers for the Linear Learner algorithm.  Since we want this notebook to run in all 4 of Amazon SageMaker's regions, we'll create a small lookup.  More details on algorithm containers can be found in [AWS documentation](https://docs-aws.amazon.com/sagemaker/latest/dg/sagemaker-algo-docker-registry-paths.html)."
+    "Now we can begin to specify our linear model.  First, let's specify the containers for the Linear Learner algorithm.  Since we want this notebook to run in all of Amazon SageMaker's regions, we'll use a convenience function to look up the container image name for our current region.  More details on algorithm containers can be found in [AWS documentation](https://docs.aws.amazon.com/sagemaker/latest/dg/sagemaker-algo-docker-registry-paths.html)."
    ]
   },
   {