Fix another mistake in a comment.

src/edwards.rs

@@ -391,7 +391,7 @@ impl ConstantTimeEq for EdwardsPoint {
         // We would like to check that the point (X/Z, Y/Z) is equal to
         // the point (X'/Z', Y'/Z') without converting into affine
         // coordinates (x, y) and (x', y'), which requires two inversions.
-        // We have that X = xZ and X' = x'Z. Thus, x = x' is equivalent to
+        // We have that X = xZ and X' = x'Z'. Thus, x = x' is equivalent to
         // (xZ)Z' = (x'Z')Z, and similarly for the y-coordinate.
 
         (&self.X * &other.Z).ct_eq(&(&other.X * &self.Z))