Fix markdown nit (#17163)

Without a leading blank line, it doesn't render properly in https://www.tensorflow.org/programmers_guide/variables#variable_collections.

Author: sourcefrog

tensorflow/docs_src/programmers_guide/variables.md

@@ -62,9 +62,10 @@ them. For this reason TensorFlow provides **collections**, which are named lists
 of tensors or other objects, such as `tf.Variable` instances.
 
 By default every `tf.Variable` gets placed in the following two collections:
+
  * `tf.GraphKeys.GLOBAL_VARIABLES` --- variables that can be shared across
-multiple devices,
- * `tf.GraphKeys.TRAINABLE_VARIABLES`--- variables for which TensorFlow will
+   multiple devices,
+ * `tf.GraphKeys.TRAINABLE_VARIABLES` --- variables for which TensorFlow will
    calculate gradients.
 
 If you don't want a variable to be trainable, add it to the