Fixes README.md responses example typo "unprocessible"

[gdiasdasilva]

This PR fixes #247.

The current responses example has a typo in what should be unprocessable_entity (currently unprocessible) according to the documentation of Plug.Conn.Status, making it to not compile when trying to directly follow the example.

Example of the issue:

  @doc responses: [
    ok: {"User", "application/json", MyAppWeb.Schema.User}
    unprocessible_entity: {"Bad request parameters", "application/json", MyAppWeb.Schema.BadRequestParameters},
    not_found: {"Not found", "application/json", MyAppWeb.Schema.NotFound}
  ]

[moxley]

Nice catch @gdiasdasilva. Thank you!