update init var docs

Author: zsdonghao

tensorlayer/layers.py

@@ -249,8 +249,8 @@ def merge_networks(layers=[]):
     return layer
 
 def initialize_global_variables(sess=None):
-    """Excute ``sess.run(tf.global_variables_initializer())`` for TF12+ or
-    sess.run(tf.initialize_all_variables()) for TF11.
+    """Excute ``sess.run(tf.global_variables_initializer())`` for TF 0.12+ or
+    ``sess.run(tf.initialize_all_variables())`` for TF 0.11.
 
     Parameters
     ----------