Code fix in std::span example

[eugenefil]

On p.41 in 17.Iterators_Containers_Alg.pdf under title "std::span":

-    std::array2<int, 3> array2 = {1, 2, 3};
+    std::array<int, 3> array2 = {1, 2, 3};
     std::span s2{array2}; // static extent

     auto array3 = new int[3];
-    std::span s3{array3}; // dynamic extent
+    std::span s3{array3, 3}; // dynamic extent

     std::vector<int> v{1, 2, 3};
     std::span s4{v.data(), v.size()}; // dynamic extent
+
+    std::span s5{v}; // dynamic extent

In hunk order:

1. Typo: std::array2 -> std::array
2. std::span can't be made from pointer, only from pointer and size like for s4
3. It turns out span can be made from vector directly, see here. Could possibly be added to the list of examples.

[federico-busato]

thanks, @eugenefil. I didn't know that std::span can be created starting directly from a std::vector. I will add it