make TensorFlow 2 examples compatible with Python 3 (#339)

tensorflow_v2/notebooks/1_Introduction/helloworld.ipynb

@@ -37,7 +37,7 @@
    "source": [
     "# Create a Tensor.\n",
     "hello = tf.constant(\"hello world\")\n",
-    "print hello"
+    "print(hello)"
    ]
   },
   {
@@ -55,7 +55,7 @@
    ],
    "source": [
     "# To access a Tensor value, call numpy().\n",
-    "print hello.numpy()"
+    "print(hello.numpy())"
    ]
   }
  ],