The curvature of a beam is given by the second derivative of the deformation equation with respect to \( x \). The deformation equation is:
\[
y = x^2 - xL.
\]
Taking the first derivative:
\[
\frac{dy}{dx} = 2x - L.
\]
Taking the second derivative to find the curvature:
\[
\frac{d^2y}{dx^2} = 2.
\]
Thus, the curvature of the beam at the mid-span is \( \boxed{2} \).