Update Tensorflow example

`initialize_all_variables` is deprecated, although it still works, and `global_variables_initializer` is the replacement.

tensorflow-notebook/README.md

@@ -27,7 +27,7 @@ import tensorflow as tf
 hello = tf.Variable('Hello World!')
 
 sess = tf.Session()
-init = tf.initialize_all_variables()
+init = tf.global_variables_initializer()
 
 sess.run(init)
 sess.run(hello)