mod_captcha_rust: fix typo in README.md

[sando38]

During compilation :captcha is the name of the application to be installed, not :captcha_nif, which leads to a failure.

The source repo also uses:

def deps do
  [
    {:captcha, "~> 0.1", hex: :captcha_nif}
  ]
end

[badlop]

Thanks!