get rid of using namespace Eigen in sample code

doc/examples/tut_matrix_coefficient_accessors.cpp

@@ -1,17 +1,15 @@
 #include <iostream>
 #include <Eigen/Dense>
 
-using namespace Eigen;
-
 int main()
 {
-  MatrixXd m(2,2);
+  Eigen::MatrixXd m(2,2);
   m(0,0) = 3;
   m(1,0) = 2.5;
   m(0,1) = -1;
   m(1,1) = m(1,0) + m(0,1);
   std::cout << "Here is the matrix m:\n" << m << std::endl;
-  VectorXd v(2);
+  Eigen::VectorXd v(2);
   v(0) = 4;
   v(1) = v(0) - 1;
   std::cout << "Here is the vector v:\n" << v << std::endl;