message: Fix comment about Montgomery curve selection
diff: //! value `A` is the smallest integer choice such that:
  //!
  //! * `(A - 2) / 4` is a small integer (`10240`).
  -//! * `A^2 - 4` is quadratic residue.
  +//! * `A^2 - 4` is quadratic nonresidue.
  //! * The group order of the curve and its quadratic twist has a large
  //! prime factor.
  //!
  