updating DIRECTORY.md

Author: github-actions

Diff for: DIRECTORY.md

@@ -694,6 +694,8 @@
 
 ## Neural Network
   * [2 Hidden Layers Neural Network](neural_network/2_hidden_layers_neural_network.py)
+  * Activation Functions
+    * [Exponential Linear Unit](neural_network/activation_functions/exponential_linear_unit.py)
   * [Back Propagation Neural Network](neural_network/back_propagation_neural_network.py)
   * [Convolution Neural Network](neural_network/convolution_neural_network.py)
   * [Input Data](neural_network/input_data.py)