Update stale example links

doc/source/example-a3c.rst

@@ -9,7 +9,7 @@ View the `code for this example`_.
 
 .. _`A3C`: https://arxiv.org/abs/1602.01783
 .. _`Universe Starter Agent`: https://github.com/openai/universe-starter-agent
-.. _`code for this example`: https://github.com/ray-project/ray/tree/master/python/ray/rllib/a3c
+.. _`code for this example`: https://github.com/ray-project/ray/tree/master/python/ray/rllib/agents/a3c
 
 .. note::
 

doc/source/example-evolution-strategies.rst

@@ -11,7 +11,7 @@ To run the application, first install some dependencies.
 
 You can view the `code for this example`_.
 
-.. _`code for this example`: https://github.com/ray-project/ray/tree/master/python/ray/rllib/es
+.. _`code for this example`: https://github.com/ray-project/ray/tree/master/python/ray/rllib/agents/es
 
 The script can be run as follows. Note that the configuration is tuned to work
 on the ``Humanoid-v1`` gym environment.

doc/source/example-policy-gradient.rst

@@ -39,4 +39,4 @@ Many of the TensorBoard metrics are also printed to the console, but you might
 find it easier to visualize and compare between runs using the TensorBoard UI.
 
 .. _`TensorFlow with GPU support`: https://www.tensorflow.org/install/
-.. _`code for this example`: https://github.com/ray-project/ray/tree/master/python/ray/rllib/ppo
+.. _`code for this example`: https://github.com/ray-project/ray/tree/master/python/ray/rllib/agents/ppo