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.
Merge pull request #2180 from MSch/docfix-special_forms-map

Docfix: Use Map.put/3 in Kernel.SpecialForms struct docs.