How can I calculate an integral of complex number on real line

Question

I'm studying a book about complex numbers. In the solution of one of the example of this book, There is a proposition which says : "For calculate $\int_1^{-1} (\zeta+1)^{- \frac{2}{3}}(\zeta-1)^{- \frac{2}{3}} d\zeta$, we can calculate this on the real line and get $\int_1^{-1} (x+1)^{- \frac{2}{3}}(1-x)^{- \frac{2}{3}} e^{-\frac{2\pi i}{3}} dx$. I want to know, how writer does get this real integral?