Fix an uninitialized variable due to a typo

Author: WardBrian

test/unit/math/prim/constraint/lb_transform_test.cpp

@@ -90,7 +90,7 @@ TEST(prob_transform, lb_constrain_std_vector) {
   Eigen::VectorXd lb(2);
   lb << 2.0, stan::math::NEGATIVE_INFTY;
   Eigen::VectorXd lb2(2);
-  lb << stan::math::NEGATIVE_INFTY, 1.0;
+  lb2 << stan::math::NEGATIVE_INFTY, 1.0;
   double lbd = 2.0;
 
   Eigen::VectorXd lb_bad(3);