Fix typo in complex exponential function

src/1_complex_numbers.md

@@ -116,7 +116,7 @@ involving cosines and sines.
 It also makes doing many common operations on complex number a lot easier.
 
 The exponential function $f(z) = \exp(z) = e^z$ is defined as:
-$$\exp(z) = e^{x + {\rm i}y} = e^{x} + e^{{\rm i} y} = e^{x} \left( \cos y + {\rm i} \sin y\right).$$
+$$\exp(z) = e^{x + {\rm i}y} = e^{x} e^{{\rm i} y} = e^{x} \left( \cos y + {\rm i} \sin y\right).$$
 The last expression is called the *Euler identity*.
 
 **Exercise** Check that this function obeys