Update instance type form c4.2xlarge to c4.xlarge due to capacity issue

Author: neelamgehlot

sagemaker-python-sdk/mxnet_gluon_sentiment/mxnet_sentiment_analysis_with_gluon.ipynb

@@ -110,7 +110,7 @@
     "m = MXNet('sentiment.py',\n",
     "          role=role,\n",
     "          train_instance_count=1,\n",
-    "          train_instance_type='ml.c4.2xlarge',\n",
+    "          train_instance_type='ml.c4.xlarge',\n",
     "          framework_version='1.3.0',\n",
     "          py_version='py2',\n",
     "          distributions={'parameter_server': {'enabled': True}},\n",