Moved relu.py from maths/ to neural_network/activation_functions (#9753)

* Moved file relu.py from maths/ to neural_network/activation_functions

* Renamed relu.py to rectified_linear_unit.py

* Renamed relu.py to rectified_linear_unit.py in DIRECTORY.md

Author: piyush-poddar

DIRECTORY.md

@@ -639,7 +639,6 @@
   * [Quadratic Equations Complex Numbers](maths/quadratic_equations_complex_numbers.py)
   * [Radians](maths/radians.py)
   * [Radix2 Fft](maths/radix2_fft.py)
-  * [Relu](maths/relu.py)
   * [Remove Digit](maths/remove_digit.py)
   * [Runge Kutta](maths/runge_kutta.py)
   * [Segmented Sieve](maths/segmented_sieve.py)
@@ -710,6 +709,7 @@
     * [Exponential Linear Unit](neural_network/activation_functions/exponential_linear_unit.py)
     * [Leaky Rectified Linear Unit](neural_network/activation_functions/leaky_rectified_linear_unit.py)
     * [Scaled Exponential Linear Unit](neural_network/activation_functions/scaled_exponential_linear_unit.py)
+    * [Rectified Linear Unit](neural_network/activation_functions/rectified_linear_unit.py)
   * [Back Propagation Neural Network](neural_network/back_propagation_neural_network.py)
   * [Convolution Neural Network](neural_network/convolution_neural_network.py)
   * [Perceptron](neural_network/perceptron.py)