get rid of using namespace Eigen in sample code

doc/examples/TutorialLinAlgInverseDeterminant.cpp

@@ -1,16 +1,13 @@
 #include <iostream>
 #include <Eigen/Dense>
 
-using namespace std;
-using namespace Eigen;
-
 int main()
 {
-   Matrix3f A;
+   Eigen::Matrix3f A;
    A << 1, 2, 1,
         2, 1, 0,
         -1, 1, 2;
-   cout << "Here is the matrix A:\n" << A << endl;
-   cout << "The determinant of A is " << A.determinant() << endl;
-   cout << "The inverse of A is:\n" << A.inverse() << endl;
+   std::cout << "Here is the matrix A:\n" << A << std::endl;
+   std::cout << "The determinant of A is " << A.determinant() << std::endl;
+   std::cout << "The inverse of A is:\n" << A.inverse() << std::endl;
 }