Update docs/source/usage.rst

Improving output clarity, according @tdegeus suggestion

Co-authored-by: Tom de Geus <tdegeus@users.noreply.github.com>

Author: tatatupi

docs/source/usage.rst

@@ -49,7 +49,7 @@ returning a ``std::tuple``, which can be unpacked using ``std::get<N>(tuple)``.
 
     xt::xarray<double> a = {{3, 2, 1}, {0, 4, 2}, {1, 3, 5}};
     auto d = xt::linalg::slogdet(a);
-    std::cout << std::get<0>(d) << ", " << std::get<1>(d) << std::endl;  // 1.0000000, 3.7376696
+    std::cout << std::get<0>(d) << ", " << std::get<1>(d) << std::endl;  // +1, 3.7376696
 
 Returning tuples is used throughout the xlinalg package.