Determinant example documentation fix

The determinant of the example is supposed to be zero, which is the output of  `xt::linalg::det(a)`. And for zero determinant matrices, the sign is 0 and logdet is -inf.

Author: tatatupi

docs/source/usage.rst

@@ -37,7 +37,7 @@ For example, calculating a determinant:
     {
         xt::xarray<double> a = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
         auto d = xt::linalg::det(a);
-        std::cout << d << std::endl;  // 6.661338e-16
+        std::cout << d << std::endl;  // 0.0
     }
 
 We can also try to compute the same determinant using the ``slogdet`` function, which
@@ -49,7 +49,7 @@ returning a ``std::tuple``, which can be unpacked using ``std::get<N>(tuple)``.
 
     xt::xarray<double> a = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}};
     auto d = xt::linalg::slogdet(a);
-    std::cout << std::get<0>(d) << ", " << std::get<1>(d) << std::endl;  // 1, -34.9450...
+    std::cout << std::get<0>(d) << ", " << std::get<1>(d) << std::endl;  // 0.0, -inf
 
 Returning tuples is used throughout the xlinalg package.