get rid of using namespace Eigen in sample code

doc/examples/tut_matrix_resize_fixed_size.cpp

@@ -1,11 +1,9 @@
 #include <iostream>
 #include <Eigen/Dense>
 
-using namespace Eigen;
-
 int main()
 {
-  Matrix4d m;
+  Eigen::Matrix4d m;
   m.resize(4,4); // no operation
   std::cout << "The matrix m is of size "
             << m.rows() << "x" << m.cols() << std::endl;