fixing small typo in FFT chapter.

Author: leios

chapters/FFT/cooley_tukey.md

@@ -46,7 +46,7 @@ $$F(\xi) = \int_{-\infty} ^\infty f(x) (\cos(-2 \pi x \xi) + i \sin(-2 \pi x \xi
 
 and our function in real space into:
 
-$$f(x) = \int_{-\infty} ^\infty F(\xi) (\cos(2 \pi \xi x) + i \sin(2 \pi \xi x))e^{2 \pi i \xi x} d\xi$$
+$$f(x) = \int_{-\infty} ^\infty F(\xi) (\cos(2 \pi \xi x) + i \sin(2 \pi \xi x)) d\xi$$
 
 Here, the $$\sin$$ and $$\cos$$ functions are clearly written in the formulas, so it looks much friendlier, right?
 This means that a point in real space is defined by the integral over all space of it's corresponding frequency function multiplied by sinusoidal oscillations.