updating DIRECTORY.md

Author: MRJPEREZR

DIRECTORY.md

@@ -828,6 +828,7 @@
     * [Squareplus](neural_network/activation_functions/squareplus.py)
     * [Swish](neural_network/activation_functions/swish.py)
   * [Back Propagation Neural Network](neural_network/back_propagation_neural_network.py)
+  * [Backpropagation Weight Decay](neural_network/backpropagation_weight_decay.py)
   * [Convolution Neural Network](neural_network/convolution_neural_network.py)
   * [Input Data](neural_network/input_data.py)
   * [Simple Neural Network](neural_network/simple_neural_network.py)