linear_solver: Add missing import region tag

ortools/linear_solver/samples/LinearProgrammingExample.cs

@@ -12,8 +12,10 @@
 // limitations under the License.
 
 // [START program]
+// [START import]
 using System;
 using Google.OrTools.LinearSolver;
+// [END import]
 
 public class LinearProgrammingExample
 {

ortools/linear_solver/samples/LinearProgrammingExample.java

@@ -13,12 +13,13 @@
 
 // [START program]
 package com.google.ortools.linearsolver.samples;
-
+// [START import]
 import com.google.ortools.Loader;
 import com.google.ortools.linearsolver.MPConstraint;
 import com.google.ortools.linearsolver.MPObjective;
 import com.google.ortools.linearsolver.MPSolver;
 import com.google.ortools.linearsolver.MPVariable;
+// [END import]
 
 /** Simple linear programming example.*/
 public class LinearProgrammingExample {

ortools/linear_solver/samples/SimpleLpProgram.cs

@@ -13,8 +13,10 @@
 
 // Minimal example to call the GLOP solver.
 // [START program]
+// [START import]
 using System;
 using Google.OrTools.LinearSolver;
+// [END import]
 
 public class SimpleLpProgram
 {