Update example to use Map.put/3, accordingly with the last proposal

[Proposal 1](https://gist.github.com/josevalim/9acd0fe43063cbc33aa6)
has the update operation also put new keys in the map, while the
[2nd proposal](https://gist.github.com/josevalim/b30c881df36801611d13)
doesn't support this. The docs reflected the first proposal.

Author: MSch

lib/elixir/lib/kernel/special_forms.ex

@@ -138,7 +138,7 @@ defmodule Kernel.SpecialForms do
   regular map operations:
 
       user = %User{}
-      %{ user | a_non_struct_key: :value }
+      Map.put(user, :a_non_struct_key, :value)
 
   An update operation specific for structs is also available: