Fix docstring of erfc (#346)

Author: devmotion

src/erf.jl

@@ -88,7 +88,7 @@ Compute the complementary error function of ``x``, defined by
 ```math
 \operatorname{erfc}(x)
 = 1 - \operatorname{erf}(x)
-= \frac{2}{\pi} \int_x^\infty \exp(-t^2) \; \mathrm{d}t
+= \frac{2}{\sqrt{\pi}} \int_x^\infty \exp(-t^2) \; \mathrm{d}t
 \quad \text{for} \quad x \in \mathbb{C} \, .
 ```