up tuto so that problem is not solved by presolve (#138)

Author: geoffroyleconte

tutorials/introduction-to-ripqp/index.jmd

@@ -20,7 +20,7 @@ c = [-8.; -3; -3]
 A = [1. 0. 1.
      0. 2. 1.]
 b = [0.; 3]
-l = [0.;0;0]
+l = [-1.0;0;0]
 u = [Inf; Inf; Inf]
 QM = QuadraticModel(
   c,