Proof of the formula for dimension of moduli of stable vector bundles on smooth curves

Question

Let $C$ be a smooth curve of genus $g \ge 2$ over an algebraically closed field of positive characteristic. If I understand correctly, the dimension of the moduli space of vector bundles on $C$ of rank $r$ and degree $d$ is given by $r^2(g(C)-1)+1$. Can someone suggest me a reference for the proof of this fact.