Updates pseudocode example usage from Mix.env to Mix.env() to avoid confusion

[AlejandroFabianCampos]

I'm just getting into Elixir, so please bear with me if this is nonsense.

I was reading through the "introduction to Mix" guide, and https://github.com/elixir-lang/elixir-lang.github.com/blame/master/getting-started/mix-otp/introduction-to-mix.markdown#L283-L289 confused me slightly, since it is referencing that Mix.env is a function, but then not executing on the example usage.

By scrolling back, I could verify that the example mix.exs snippet https://github.com/elixir-lang/elixir-lang.github.com/blame/master/getting-started/mix-otp/introduction-to-mix.markdown#L85-L114, effectively points to executing Mix.env as function.

Sorry if this comes across like a little bit nitpicky, just trying to contribute back my two cents, love the docs!

[josevalim]

This is really good and the code formatter would add those parens too. Thank you!

[josevalim]

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