Update numerical_methods/fast_fourier_transform.cpp

Co-authored-by: David Leal <halfpacho@gmail.com>

numerical_methods/fast_fourier_transform.cpp

@@ -24,6 +24,11 @@
 #include <iostream>  /// for IO operations
 #include <vector>    /// for storing test cases
 
+/**
+ * @namespace numerical_methods
+ * @brief Numerical algorithms/methods
+ */
+namespace numerical_methods {
 /**
  * @brief FastFourierTransform is a recursive function which returns list of
  * complex numbers